How do I untune a Thorn TV model CT5122T?

I gave my old TV to my dad and had to re-tune it without a manual. I eventually realised how to tune it in but not before I'd accidentally tuned 3 stations in on AV. Now I can't untune them so I have 2 stations working perfectly and 3 showing black screens. The AV channels seem to be locked and I don't know how to unlock them. Can anyone help?