User Defined Function Add-in, Can I edit?

[Certified]

I created a user defined function in a workbook and saved the workbook as an add-in. The function is operating perfectly, but how do I make changes to the function (add-in)? I tried to make changes, but when I saved the file the changes didn't stick. And now I can't open the file because I keep getting a message the reads "Sorry, Excel can't open two workbooks with the same name at the same time."

I checked, I don't show the workbook open at all.

Any thoughts?