Marriage, according to the law of this country, is the union of two people of the same or opposite sex, voluntarily entered into, for life, to the exclusion of all others. 

This is made by a verbal contract between two people.  The contract is made by the couple saying declaratory and contracting words.  A ceremony is required and a marriage schedule is signed after the legal declaratory and contracting words have been spoken. 

You cannot currently convert a marriage to a civil partnership. 

A civil partnership is a written contract formed by declaration between two people giving the partners similar legal rights as married couples.  The declaration can be said out loud or read by the partners before signing.  No ceremony is required although one can be held after the declaration has been made.

Same and opposite sex partners have been able to form a civil partnership since 31 December 2019.

Same sex partners can currently convert a civil partnership to a marriage.

We recommend you seek independent advice if you are not sure whether a marriage or a civil partnership best suits your wishes.

A comparison can be found here but is subject to update on the recent legislation changes:

A comparison of civil partnerships and marriage