git replace local version with remote version

I understand the question as this: you want to completely replace the contents of one file (or a selection) from upstream. You don't want to affect the index directly (so you would go through add + commit as usual).

Simply do

git checkout remote/branch -- a/file b/another/file

If you want to do this for extensive subtrees and instead wish to affect the index directly use

git read-tree remote/branch:subdir/

You can then (optionally) update your working copy by doing

git checkout-index -u --force

This is the safest solution:

git stash

Now you can do whatever you want without fear of conflicts.

For instance:

git checkout origin/master   # or origin/main

If you want to include the remote changes in the master branch you can do:

git reset --hard origin/master  # or origin/main

This will make you branch "master" to point to "origin/master".