2degrees: move from Pay Monthly to Prepay

I am currently on Pay Monthly and find myself not using that much data, and want to switch to Prepay.

I want a self-service way to do this via web or the 2degrees app. Can anyone tell me how can I do it? I tried to logon to my account using my PC, and try to look for that option in the app, but cannot find anything useful. Also, can I request to switch to Prepay on the last day of the bill cycle of my Pay Monthly plan?

Thanks!