Say, you have main & feature as 2 branches.

Merge conflicts

If you want to merge feature branch into main, run following command.

git checkout main
git merge feature

This can result in merge conflicts.

To discard all changes in feature and accept everything on main, run following command.

git merge -Xours feature

To accept changes from feature branch, run below command.

git merge -Xtheirs feature

Rebase conflicts

If you want to rebase feature branch onto main, run below command.

git checkout feature
git rebase main

Again, this can result in merge conflicts.
To accept the changes in feature branch, run below command.

git rebase main -Xtheirs

To accept the changes in main branch, run below command.

git rebase main -Xours