To upgrade to the latest version (if you have one already installed but need to update the version) do the following:
1. brew update
2. brew unlink maven
3. brew install maven
if you then execute the mvn --version command you should see that your version of maven has been upgraded.
EDIT:
If you have an issue with the update (command 1) that goes along the lines of
error: The following untracked working tree files would be overwritten by merge:
Library/Formula/...
try this answer from another issue