If you own the flat, just call your energy supplier and they should be able to send someone out to swap the meter.
If you rent, then in theory the process is the same, but you might want/need to notify your landlord. As long as you don't rack up unpaid bills then it shouldn't affect them, but they might want you to get it swapped back to pre-payment when you move out.
We did the same thing when living in a Council Flat and we had to coordinate with the Council and NPower to come on the same day as the Council has to give access to the electrics cupboard for isolation - this probably isn't located inside your flat.
As it turns out the cupboard in our particular flat didn't need the Council, it just needed a screwdriver, you may want to check if this is the same for yours as it will save a lot of hassle on your part! It took three attempts before it was finally changed!