How to stop my universal credit

So basically im starting a course and i want to tell them and get it sorted without having to ring them as im petrified of phone calls ... how does one do that ?

Write a letter and hand it in. If you have a book then there might be a form in there to simply return and end your claim.

I rang them and they wanted soo much information i always got soo anxious on the phone with them and they said they would send me a letter if it affects my money ... which i obviously will why couldn't they have closed the claim there and then agh its soo awkward