A simple codicil will suffice:
"This is the first codicil to the will dated . . . . . of me David Sunny Smith of 23a Acacia Avenue, Answerbankville, in the County of Borsetshire.
1. I revoke my appointment of Buen Chico as executor if either [Executor 1] or [Executor 2] shall predecease me.
2. I appoint Bar Maid of 14 Luxury Avenue, Peterborough, in the County of Cambridgeshire as executor if either [Executor 1] or [Executor 2] shall predecease me.
3. In all other aspects I confirm my will.
Dated 3rd November 2013.
[Signature]
Signed by David Sunny Smith in our presence and then by us in his:
[Signature of Witness 1, with his/her name and address printed underneath]
[Signature of Witness 2, with his/her name and address printed underneath]"
Any adult (who is not a beneficiary of your will) can witness the codicil. However it's important that they actually see you sign it. (If, for example, you sign it and then take it round to your neighbours' house to ask them to act as witnesses, their signatures won't be valid. They must SEE you sign the codicil). My will was witnessed (by a couple of people I happened to be working with) in a Tesco car park!