Default [Q] Apn settings

I have a galaxy s5 900f. On contract with O2. I recently got a giffgaff pay as u go sim.
When I switch the sim back to giffgaff the apn settings haven't been saved.
Any ideas how to save the apn settings.Apologies if this has been asked already.
Phone:Samsung Galaxy S, S3, S5
Model No: GT-I9300, 900F