Update a Addin to work on Excel 365

Hi I have an addin I purchased many years ago to run a statistical analysis. It works earlier versions of excel before the some of the file formats changed for VBA. I would like the addin improved to be able to work in Excel 365 and current file formats.

Currently i get a Run-time error '9' Subscript out of range.

I developer who originally created the addin is no longer around.

Happy to discuss cost and time required to achieve this work