Assuming you passed your test before 1 January 1997 you will automatically have category C1 entitlement on your licence, allowing you to drive vehicles up to 7,500kg. You will need to fill in form D2 and ask your GP to fill in a medical report form D4
. Your GP is likely to charge for this service as part of a private appointment.
If you have the reminder form, you can use that to apply and if you don't have it, you can get one from a post office. It cannot be renewed online as your standard licence can.
I found this, and a lot more, information on The Camping and Caravanning Club website:
Most GPs charge quite a lot for the medical and my friends on Trucknet suggested this website to find a doctor who will charge a lot less. I used it before I retired (I was an HGV driver) and the "medical" is strictly confined to the requirements of the DVLA. Blood pressure, Eyesight and answering yes or no to a long list of questions.