Yes, any Template you own you can edit. (Templates shared with you can be edited as well if you have specific permissions). If you already have a document on a Template and wish to switch the current document with a new document, you should use the More Options > Replace (aka three dots in the bottom right of the current document in edit mode).  The “replace” option will allow you to switch out the old document with a new document without loosing all the existing field sand then you can add or move fields as needed.

David.  My template is a coversheet that I need to submit with each envelope I create and the coversheet needs to be signed by several people.  I also want to add a 2nd document to that envelope and in that 2nd document, I want to add signature boxes and/or dates.  Is that possible?  Thx, Paul

@Paul Morris Yes, you can start an envelope and add a Template or “use” a Template and add a new document. You can also “Use” a Template, add a document, then apply a Template to the new document. There are many variations that can be used within DocuSign if you have the ability to create your own Templates to be more efficient with your envelope sending. Here is an article starting point.

https://support.docusign.com/s/document-item?bundleId=gbo1643332197980&topicId=aoz1578456267317.html&_LANG=enus

Basically I would categorize Template into Static and Dynamic (overlay).  With static Template you add a specific document and set of Recipients and set of fields and that is “Used” over and over to create the same envelope experience.  Dynamic are more applied based on need and different dynamic Templates can be applied based on the length or possibly the number of Recipients.  So perhaps sometimes you document is 5 pages and sometimes 10 and may vary in number of Recipients.  You can create several Templates as dynamic overlays so you can apply the right one based on the the situation and requirements.  

In your case I would create a standard Template that has all the basics that must be present with each envelope and then create other Templates that can be added or applied over documents that might need to be added based on the situation.