How do you change joint account to single account?

It depends what country you're in... Here in the UK - the person wanting to be removed from the account has to provide a written declaration that they no longer wish to have access to the account.. This then needs to be signed at the bank, in front of a bank employee - and both people need to provide proof of their identities - along with specimen signatures. The bank will then remove the person from the account, and restrict access to the single signatory.