I have not seen this before - you should be able to edit directly "if" you added the envelope to you document which I automatically do.
I will create the letter, highlight the address then create the envelope "Tools/Envelopes". Before closing the dialogue I select "Insert this envelope into the active document" where it adds it on its own page at the beginning of the document. Then, I can move or alter the address as I wish. When you want to print click into the envelope page select "File/Print" and select print "current page".
I keep a document like this on my desktop so I can print adhoc envelopes quickly and easily.
Hope this helps.
Alan