How can I mark one file as unresolved after a failed merge in git?

Question

At the moment I am uniting a very large project and, unfortunately, I have a lot of conflicts. After merging some files, I realized that I had merged errors in a specific file, and now it is marked as resolved. How can I deploy it to git to merge it again.

I tried git reset --hard -- myFile.txt, but it does not work.

Regards, Steffen