Spend a lot of time looking for this one, here what I've found so far:
Change default iis express port
change default iis for web application
I have a web site in visual studio 2015 (aspx) I want to change the default port. No way to do it. Here are some hints - that used to work, or works for web application but this is not the case. no csproj file, changing applicationhost.config did not help either.