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Try the button on this link to download and install Prime 11.:https://www.mathcad.com/en/try-and-buy/mathcad-express-free-download/install
Note that you don't 'upgrade', but you'll get Prime 11 installed next to Prime 10.
Following installation you should be able to apply for, and get the 30-day trial for Prime 11. Use the "Get license" button on the Options form of the application for that.
Note that the Prime 11 in express mode does not give you more capabilities than Prime 10 Express, other than being able to open Prime 11 files.
Success!
Luc