Have you tested 'Fetch upstream' when in your forks 'master' branch?
If that doesn't sort it for you, I'd say do:
Create a new branch in your repo 'mybackup' from the branch you've made commits/changes to ('master' in your case) . (Select branch 'master' -> then enter 'mybackup' in the text input field -> 'Create branch: ...')
Or/and download your fork repo as zip.
Set up git and gh (github) in your terminal/command line.
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Same! Worked this out just last night.
Have you tested 'Fetch upstream' when in your forks 'master' branch?
If that doesn't sort it for you, I'd say do:
Create a new branch in your repo 'mybackup' from the branch you've made commits/changes to ('master' in your case) . (Select branch 'master' -> then enter 'mybackup' in the text input field -> 'Create branch: ...')
Or/and download your fork repo as zip.
Set up git and gh (github) in your terminal/command line.
Take inspiration from this medium article: How to reset your git branch to a previous commit (both local and remote)
Maybe keep your 'master' branch pristine and create a new one where you work on your 'subtle changes'.
Get files you added/modified from backup and paste them back into your new branch (or 'master' branch).
There may be a better way to go about it - but this worked for me.