Currently it is not possible to add additional information or reformat the system generated spay and neuter certificates.

If you would like to add additional information, such as post operative instructions,  you could insert it as a medical or pop-up memo with a date for when post-op healing is done and bathing/activity can resume after the animal is spayed/neutered. Medical memos appear at the top of the Complete Medical History Document, so it is easily accessible to staff and adopters if they are provided with this document at checkout. Pop-up memos will appear at the top of an animal's record and may be helpful in reminding staff to inform adopters of this information. The Memos Document can also be printed or emailed for an adopter. 

Alternatively, you could add post operative instructions to the 'Stored text' section within the surgery or procedure. This information will print on the Complete Medical History Document.