Please upgrade here. These earlier versions are no longer being updated and have security issues.
HackerOne users: Testing against this community violates our program's Terms of Service and will result in your bounty being denied.
How to replace a string via MySQL command on all discussions?

Answers
You replace things in MySql by using MySqls replace command
https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_replace
You would have to replace in the column "Body" of the tables GDN_Discussion and GDN_Comment