Under the Texas Probate
Code, in order to modify the clause designating the executor, you would need to add a codicil to the will revoking the original clause which designated the executor in favor of the new clause designating a different executor. The codicil would need to be signed by the testator and witnessed in the same manner as the will. The relevant section of the Texas Probate Code is copied below:
§ 63. REVOCATION OF WILLS. No will in writing, and no
clause thereof or devise therein, shall be revoked, except by a
subsequent will, codicil, or declaration in writing, executed with
like formalities, or by the testator destroying or canceling the
same, or causing it to be done in his presence.
Good luck. I hope this is helpful.