As I have downloaded the ZIP file, is it possible to install the board from it?
Error MSg
Error downloading https://github.com/esp8266/Arduino/releases/download/2.6.3/esp8266-2.6.3.zip
java.lang.RuntimeException: java.lang.Exception: Error downloading https://github.com/esp8266/Arduino/releases/download/2.6.3/esp8266-2.6.3.zip
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:175)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.Exception: Error downloading https://github.com/esp8266/Arduino/releases/download/2.6.3/esp8266-2.6.3.zip
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:144)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:78)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:60)
at cc.arduino.contributions.packages.ContributionInstaller.install(ContributionInstaller.java:105)
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:172)
... 1 more
Caused by: java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(SocketInputStream.java:210)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at sun.security.ssl.InputRecord.readFully(InputRecord.java:465)
at sun.security.ssl.InputRecord.readV3Record(InputRecord.java:593)
at sun.security.ssl.InputRecord.read(InputRecord.java:532)
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:975)
at sun.security.ssl.SSLSocketImpl.readDataRecord(SSLSocketImpl.java:933)
at sun.security.ssl.AppInputStream.read(AppInputStream.java:105)
at java.io.BufferedInputStream.read1(BufferedInputStream.java:284)
at java.io.BufferedInputStream.read(BufferedInputStream.java:345)
at sun.net.www.MeteredStream.read(MeteredStream.java:134)
at java.io.FilterInputStream.read(FilterInputStream.java:133)
at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3444)
at sun.net.www.protocol.http.HttpURLConnection$HttpInputStream.read(HttpURLConnection.java:3437)
at cc.arduino.utils.network.FileDownloader.readStreamCopyTo(FileDownloader.java:268)
at cc.arduino.utils.network.FileDownloader.openConnectionAndFillTheFile(FileDownloader.java:247)
at cc.arduino.utils.network.FileDownloader.downloadFile(FileDownloader.java:182)
at cc.arduino.utils.network.FileDownloader.download(FileDownloader.java:129)
at cc.arduino.contributions.DownloadableContributionsDownloader.download(DownloadableContributionsDownloader.java:142)
... 5 more