People already in a registered civil partnership are able to make an application to convert from civil partnership to marriage.
This can be done either by means of a marriage ceremony (civil, religious or belief) or by the administrative route.
The administrative route is to visiting your local Registrar's Office with your civil partnership certificate, completing the application form (which must be signed in the presence of a Registrar and also in front of each other) and producing the required photographic identification (Passport, Driver's License).
The fee for the administrative conversion is now £30, and a fee of £10 is payable for the Marriage Certificate. For guidance on this please see Guidance on changing a civil partnership to a marriage.