I went on the internet and I found this:
To alter the license. First, go to
Windows: %APPDATA%\syntevo\SmartGit<main-smartgit-version>
Windows portable: SmartGit\.settings\preferences.yml
OS X: ~/Library/Preferences/SmartGit/<main-smartgit-version>
Unix/Linux: ~/.smartgit/<main-smartgit-version>
and remove the file settings.xml.
If you have updated many times, you may need to remove the updates folder as well.
It helped me on Windows, hope it helps you on other systems as well.