You need to re-test (need to do atleast sanity test) the whole application, because you may not know some times, the one which you have modified is going to effect for other modules in the application.
Actually it depends on the module you are modifying. If this module have no impact on other modules, you don't have to re-test entire application.
If the change in your module have functionality association with other modules, you need to test all those modules as well.
its enough to just re-test functinality associated with module.nt neccesary to re-test whole application.