Changing a Hager MCB

Hi guys...newbie here!!!

I have just had a new electric oven installed by John Lewis. The guy installing it has told me that I need to change the 32A MCB in the CU to either a 16A or 20A MCB before I can operate the oven.

The MCB currently in there is a Hager MCB (Type B) Rated 32 Amps, (it has MTN132 B32 on it).

Would it be easy for me to change it myself or do I need to get a sparky in?
If it is something I could do myself, would you advise 16A or 20A?

thanks for any help you can give.....