Check to see if your present Sim can Pop-out a,"Nano Sim".
You can order sim-Triple Pack free.
I'm too impatient to wait for a free Sim so..
buy one for less than a pound at Wilkos/B&M/Poundshop/Tesco/Argos/Iceland/ASDAand even my corner shop sells them now and do a,"Sim Swap".Most of these places are open until 8pm Monday to Saturday and 10am til 4pm Sundays/Bank holidays.Do NOT activate the New Sim the normal way e.g don't put any money on it
as a blank unused Sim is needed for a Sim swap and the Sim-Swap process will activate the new Sim.
and here's an easy to follow video,
you can only do a SIM swap between 4.30am and 9.30pm and it can be like when you first join GiffGaff
e.g keep restarting the phone over a few hours after the Sim Swap.
This will also transfer all your number/calls/text and data allowance across and disable the old Sim-Cardso it can't be used.
Or you can go to any local Phone shop(Not Carphone Warehouse as they charge too much) and they have templates to cut the sim down for around a pound.
Or if you want you can have a go yourself to cut one down,