What is a system app?
System apps are usually apps installed as part of the #dic:operatingsystem. They can’t easily be removed or updated by the user, but may get updated along with OS updates.
Replacing a system app
If you want to use a different app for the same purpose as a system app (e.g. browser or mail app) you can usually disable the system app in Settings -> Apps and or set your preferred app as the default app for that purpose (depending on the OS you are using) in Settings -> Apps -> .