How do I update the mailmerge buttons and add new ones to the opportunity?

Hi there! I've recently updated all our thank you letters that we send out to donors. 

Each opportunity in lightening has a series of mail merge buttons in salesforce that link with the letters so if I only want to send 1 letter in the opportunity I can just click on it and the merge action runs. 

How do I update the merge action?  How do I add more buttons with new logic and how do I attach the new letter to the merge button?  Thanks so much!