I ended up with multiple commits in my PR after a rebase (and followup push) in my "working" branch.
Solution:
I first copied those few files which I have modified/ added into a temp directory (along with entire path).
then reset my branch with origin/master.
git reset --hard origin/master
So, now the "working" branch is in sync with master branch.
Now, copy back all those files from temp directory into the "working" branch.
Carried out a fresh commit with all changes and verified all changes looks fine.
Then, just did a force push as :
git push --force
Now, you can check in github whether only one commit with required changes present.
Lesson:
Always verify git status before a push especially after a rebase.
PS: not an optimal solution, but manual way.