I am currently trying to upgrade a Jenkins build server that has multiple slave servers. 
The upgrade is from version 2.46.1 to 2.263.2 (so quite a few versions different)

It looks like it is unable to download the war file. 
I am able to download the war file from the Jenkins server manually just fine using the url. 
https://updates.jenkins.io/download/war/2.263.2/jenkins.war
I attached 2 screenshots and the full error message I'm getting.

Any help/advice would be greatly apricated!

 
Failure -
sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target at sun.security.provider.certpath.SunCertPathBuilder.build(SunCertPathBuilder.java:146) at sun.security.provider.certpath.SunCertPathBuilder.engineBuild(SunCertPathBuilder.java:131) at java.security.cert.CertPathBuilder.build(CertPathBuilder.java:280) at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:382) Caused: sun.security.validator.ValidatorException: PKIX path building failed at sun.security.validator.PKIXValidator.doBuild(PKIXValidator.java:387) at sun.security.validator.PKIXValidator.engineValidate(PKIXValidator.java:292) at sun.security.validator.Validator.validate(Validator.java:260) at sun.security.ssl.X509TrustManagerImpl.validate(X509TrustManagerImpl.java:324) at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:229) at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:124) at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1491) Caused: javax.net.ssl.SSLHandshakeException at sun.security.ssl.Alerts.getSSLException(Alerts.java:192) at sun.security.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1949) at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:302) at sun.security.ssl.Handshaker.fatalSE(Handshaker.java:296) at sun.security.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1509) at sun.security.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:216) at sun.security.ssl.Handshaker.processLoop(Handshaker.java:979) at sun.security.ssl.Handshaker.process_record(Handshaker.java:914) at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:1062) at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1375) at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1403) at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1387) at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:559) at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185) at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1513) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441) at sun.net.www.protocol.http.HttpURLConnection.getHeaderField(HttpURLConnection.java:2943) at java.net.URLConnection.getHeaderFieldLong(URLConnection.java:629) at java.net.URLConnection.getContentLengthLong(URLConnection.java:501) at java.net.URLConnection.getContentLength(URLConnection.java:485) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getContentLength(HttpsURLConnectionImpl.java:398) at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1123) Caused: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) at java.lang.reflect.Constructor.newInstance(Constructor.java:422) at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1890) at sun.net.www.protocol.http.HttpURLConnection$10.run(HttpURLConnection.java:1885) at java.security.AccessController.doPrivileged(Native Method) at sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1884) at sun.net.www.protocol.http.HttpURLConnection.getInputStream0(HttpURLConnection.java:1457) at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1441) at sun.net.www.protocol.https.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:254) at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1124) Caused: hudson.util.IOException2: Failed to download from https://updates.jenkins.io/download/war/2.263.2/jenkins.war at hudson.model.UpdateCenter$UpdateCenterConfiguration.download(UpdateCenter.java:1172) at hudson.model.UpdateCenter$DownloadJob._run(UpdateCenter.java:1678) at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:1652) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at hudson.remoting.AtmostOneThreadExecutor$Worker.run(AtmostOneThreadExecutor.java:110) at java.lang.Thread.run(Thread.java:745)