You should be able to change the port # of IIS Express. You can do this by entering IIS Manager, and right click on the web site you want to configure, and click "Edit Bindings".

Change the port # from 80 as shown in the photo below to something like 23478 or whatever you want that isn't in use. You can find more information on the Microsoft Knowledgebase here: http://support.microsoft.com/kb/149605

Once you edit the port # that IIS Express is running on, you will have to specify it in your Visual Studio project, so the URL will be "http://localhost:23478" or whatever port instead.
EDIT:
The above description is how to change the port # of regular IIS server. In order to change the port # of IIS Express, according to https://msdn.microsoft.com/en-us/library/ms178109.ASPX follow these steps:
To specify a port for the ASP.NET Development Server
In Solution Explorer, click the name of the application.
In the Properties pane, click the down-arrow beside Use dynamic ports and select False from the dropdown list.
This will enable editing of the Port number property.
In the Properties pane, click the text box beside Port number and type in a port number.
Click outside of the Properties pane. This saves the property settings.
Each time you run a file-system Web site within Visual Web Developer, the ASP.NET Development Server will listen on the specified port.