You can also change the port your local Mendix app is runnning.
In you Project Explorer in the Modeler go to Settings
Then, on the configurations tab edit your configuration (by default this is the configuration "Default")
Go to the Server tab, here you can change the runtime and admin port. Note that both ports have to be available on your local machine, both ports can therefore also never be the same.
Now close all popups and run.