If you are referring to severance of a joint tenancy of a freehold, then it is a simple matter of a letter addressed to your co-tenant saying "I, [name] hereby sever the joint tenancy over the property known as [property], registered title number [title number] with immediate effect. Signed and dated. It should then be acknowledged by the co-tenant and filed at the Land registry. You would then be tenants-in-common, which has implications as regards title passing on death. Best to check with a lawyer, really if you want to be sure.