As long as your project was in a 3.50 or later version, you could download and open the latest version, then right click AdvancedHMI project in Solution Explorer, select Add Existing, then browse to the forms of your previous solution.
The bad news is that version 3.80 will not be such a graceful upgrade. Some significant changes had to be made to the solution structure in order to make it easier to add C# projects. This will make it take a little more effort to upgrade to it.