To change the tax class of your vehicle, you'll need to apply at a post office (one that handles tax). You'll need the V5C in your name (if not, you'll need to complete a V62 form and pay £25). You should also take along form V11 if you have it. Your car is MoT exempt so you'll need to fill in a V112 form. You can download and print the forms from the DVLA website or request that they are posted out to you. If you don't have access to the internet at home, your local library or a family member should be able to help.