You can not merge with local modifications. Git protects you from losing important changes. You have three options.
One is to commit the change using
git commit -m "My message"
The second is to stash it. stashing acts as a stack, where you can push changes, and you pop them in reverse order.
To stash type:
Do the merge, and than pull the stash:
git stash pop
The third options is to discard the local changes using
git reset --hard
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