Solution 1:
Type out a new will, simply copying the old one and replacing the executor. (Remember to change the date!). Sign and date it in the presence of two witnesses (who must not be beneficiaries of the will) and get them to sign it.
Solution 2:
Type out a codicil as follows:
"This is the first codicil to the will dated [date] of me [full name] of [full address].
1. I revoke the appointment of [name of solicitor] as executor and appoint [name of new executor] as executor in his stead.
2. In all other respects I confirm my will.
Date:
Signature:
Signed by [full name] in our presence and then by us in his:
[Signature of 1st Witness] [Signature of 2nd Witness]
[Printed name & address] [Printed name & address]"
(Note: If you don't need to appoint another executor, simply omit "and appoint [name of new executor] as executor in his stead")