In order to prepend a string when you are updating a columns is kind of easy, you only need to know the right Mysqsl function to achieve that.
Example:
For any reason, someone requested you to update the user user_login for the user ID:999 with this symbol "@" prefix to each name:
update users
set user_login = concat('@', user_login)
where id = 999;
As you can see the function "concat" is prepending the @ to each user login.
The expected output would be
| ID | user_login |
| 999 | @john.doe |
It is important to note that Concat allows you to add a string either at the end or at the beginning.