On clicking on 'Preview', I see that Barmaid has already answered your question. However, as I've already typed out my answer, I'll post it anyway ;-)
Write the codicil as follows:
"This is the first codicil to the will dated (date of will) of me (your full name) of (your address).
1. I revoke the appointment of executors.
2. I appoint as executors (name of first executor) of (address of first executor) and (name of second executor) of (address of second executor).
3. In all other respects 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:
signatures of both witnesses (with their addresses underneath)"
Ensure that your witnesses actually see you sign and that neither of them are named as beneficiaries in your will. That's it.
Chris
(Former MD of a will-drafting company)