This question actually shows that you are not a programmer.
1. Find the word "by q2apro" in one of the PHP files.
2. Remove it.
3. Upload the edited file to the server.
PS: Lucky you, I have just removed the entire footer from the file. Now it's gone. Download the new file here: https://github.com/q2apro/q2apro-on-site-notifications/blob/master/q2apro-onsitenotifications-page.php
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April 20, 2021: Q2A 1.8.6 released!