How to solve merge conflict in git cli?

$  git merge TEST_2
 Auto-merging test.txt
 CONFLICT (content): Merge conflict in test.txt
 Automatic merge failed; fix conflicts and then commit the result.

$git mergetool --tool-help
'git mergetool --tool=' may be set to one of the following:
araxis
vimdiff
vimdiff2
vimdiff3
...

$ git mergetool --tool=vimdiff