Accidentally added a one-time add-on for my clients subscription, i need to change it to recurring


My client has a recurring subscription, with a web hosting plan, and 2 add-ons. Both are supposed to be recurring.
But one of them accidentally got used as a one-time instead of recurring.
I don't even have that add-on as recurring, so i actually don't know how that happened.
Either way, i need to change it to recurring as the rest of the invoice, how can i change it?