Log in to the Karaf console, then run:
bundle:list org.openhab.ui
Remember the bundle ID (three digit number).
Then run:
bundle:update BUNDLE_ID https://ci.openhab.org/job/openHAB-WebUI/lastSuccessfulBuild/artifact/bundles/org.openhab.ui/target/org.openhab.ui-5.1.0-SNAPSHOT.jar
This will update Main UI to the latest build from our Jenkins CI server – no openHAB restart required.