Method #1 = Click the x3 dots in the top right of the thread and then click 'edit thread'; click the status arrow ahead of the thread title and change it to [SOLVED]
If you can't do that...
Method #2 = Click the report button underneath a post in said thread and ask for it to be marked solved. A moderator will then do it in due course.
Neither necessary in this instance because I've seen it and changed the status myself.