Take it out and carefully pull the wool out from around the roots (assuming there is wool in the plastic pot) – that wool can contain fertilizers (or the seller tank water) that you don’t want to add to your tank.
To attach it, you can use thin fishing line, or thread or hair elastics. Black is a nice hidey colour for the thread or elastics – the ‘snag free’ hair elastics stretch a lot so just get the small ones. I’ve used all 3. I prefer the fishing line, but sometimes tying it on an odd shaped rock is too challenging, it keeps slipping off, and then the elastics work well.