Correct spelling, explanation: this form is the correct one because the word amend comes from Middle English amenden, and earlier French amender, both spelled with one m. There was no need to add another m, so amend is the correct form, and ammend is incorrect.

Definition of amend:
verb, to rephrase, add something or change it; usually about law
The parliament is planning to amend this Act.
We need to amend the rules of the school.

Incorrect spelling, explanation: this spelling, ammend is incorrect, because there is no reason to add another m to the root word. The word amend comes from amenden used in Middle English, and nothing points to the need of adding another m. Therefore the correct form is amend.