View => Tool Windows => Version Control. (Windows (Alt + 9) / OS X (Cmd + 9))
IntelliJ 2017.1 and higher => Go to Log and right click + reword or press F2.

While you are on the same branch, ( your checked out branch is the same )
View => Tool Windows => Version Control. (Windows (Alt + 9) / OS X (Cmd + 9))
IntelliJ 2017.1 and higher => Go to Log and right click + reword or press F2.

While you are on the same branch, ( your checked out branch is the same )