If there is a genuine reason why your guarantor needs to be changed, for example, due to a change in their financial circumstances, then you should call your lender to discuss options. Occasionally, lenders may allow you to replace your guarantor as long as the replacement is suitable. This means they will have to have similar financial circumstances to the former guarantor.
You can also change your guarantor early on in the loan agreement process. The borrower has 14 days after the loan is completed to cancel the loan. If they do choose this option, they simply have to pay back any money sent to them within 30 days and the loan is cancelled. You can then start a new loan agreement with a different guarantor.