Fifteen quid here
https://www.legalo.co.uk/template/wills/codicil-to-change-executor/
or just use the following wording:
"This is the first codicil to the will dated [date of your will] of me [your full name] of [your address].
1. I revoke the appointment of [name to be removed] as executor.
2. I appoint [name of new executor] of [address of new executor] as executor.
3. In all other aspects I confirm my will.
Date: [Date of codicil]
Signature: [Your signature]
Signed by [your full name] in our presence and then by us in his:
[Signature of first witness] [Signature of second witness]
[Printed name of first witness] [Printed name of second witness]
[*Address of first witness] [*Address of second witness]
[*Occupation of first witness] [*Occupation of second witness]"
The items marked with asterisks aren't legal requirements. (Indeed, the printed names aren't either). However they're often considered 'good practice'.
[Many, many years ago, I was the MD of a will-writing company].