If you are talking about two different versions of your app (one as a free trial and one full paid) then there is no real "easy" way to do this, you just have to produce and maintain two versions of your app.
You could consider making one version of your app and adding an in-app purchase to unlock the app content only giving access to trial content beforehand.
There are also features in both the AppStore and Play Store to provide a "free trial period" on subscriptions, if you are considering subscriptions.