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Where can I find the download link for downloading the m045 update to 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