When I download my previous year return, my windows PC attempts to open the file with MSWord and fails. How do I open the file for amendment?

Did you download the PDF of your return or the *.tax2017 data file?  You may want both, so you can look at the PDF, but you will need the *.tax2017 data file to open with the 2017 software (the "amend software" is free.)

If a file downloads as a MS document then are you using Edge?   Change the file extension to the proper one (PDF or *.tax2017 depending on the file type).  Or try using a different browser to download.

To amend you need to download the tax file ending in .tax2017  and have the Desktop program or amend program installed to open it.

What year?  How to Amend all years

https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-filed/00/25588