Use git rebase -i to enter interactive rebase mode.
In the first commit, replace pick with e to edit at that particular point and exit nano.
Now make changes in the file and commit using git add file.txt and git commit --amend -m "ok"
When we try to use git rebase --continue we encounter a merge conflict.
Simply fix that merge conflict by replacing the text in file.txt with
Hello world
Hello world is an excellent program
Add it to the staging area using git add file.txt and then use git rebase --continue to go back.