Thanks for your comments and help. But that does not answer my question .. I asked how can this be done via the current GH GUI, and not with any command line commands.
Here is the correct way (I made by myself just a minute ago) via the GH UI only:
Either fork the desired repository or branch in desired repository OR if you already forked that in the past > update it if it's behind the original
Create/edit file(s) now in your own branch to make changes
Go to the overview and select the target branch you made the the commits before
Click on the line saying "This branch is x commits ahead of xxxxx"
Click on the green button saying "View pull request"
The new PR with all changes will then be created in the target repository
Just one note: do not delete at GH your files until they are merged!
While if you have made the whole work on your PC and submitted everything to GH, deleting those files locally is allowed and has o effect to and in GitHub.