I believe I’ve managed to fix an issue in the engine where UMG Widgets are not destroyed for players on Server Travel / leaving/opening a map. The code is here: A new, community-hosted Unreal Engine Wiki - Announcements and Releases - Unreal Engine Forums
I’m not sure if in that form it’s suitable for Engine code, but if it is, I’d like to submit it as a Pull Request. Trouble is, I haven’t the first clue where to start. How can I go about uploading the fix and submitting it?
Normally you would do a check out of the master branch to make sure you base it on the latest code, make your adjustments and create a pull request for the changes.
There is nothing unreal engine specific to the actual pull request process, so any documentation on how to do one on github should be applicable.
Okay cool, thanks 
Remember to create a new branch (eg: Jamsh_UMG_Fix) instead of submitting straight from your master. Otherwise in future if you make some new changes and upload to your master that changes will appear in your UMG fix.
I made that mistake. 