Where can I find the link to download the update file m045 for mathcad 15 running on Windows 10?

The free trial

http://www.ptc.com/engineering-math-software/mathcad/free-trial/thank-you

should be same as the full version. Its just limited as of the missing license which you have to provide from your old version.

If you have sufficient rights you may try this link: https://www.ptc.com/appserver/auth/it/esd/index.jsp