Cancel sim only contract and put me on pre-paid

Can someone please help me cancel my contract as soon as possible and put my number on prepaid because the sales agent tomd me if I take this sim only Contract I would be able to get a device after one month well that didn't happen I was declined for a device so I do not see a purpose to continue this sim only contract since I was given the wrong information please cancel it effective immediately and put me on pre-paid