Online Word Quiz, Image Guess, Sound Guess Puzzle Upgrading guide from v2.0 to v2.1: To integrate this update to your existing source code, please replace the following files: - Security.java - CategoriesActivity.java - BaseActivity.java - Strings.xml (Purchase is Pending. Please complete Transaction) - Delete the 2 packages of billing by right click>> Delete >> Delete as the screenshot below : https://firebasestorage.googleapis.com/v0/b/puzzles-ccccb.appspot.com/o/Online%20app%2Fv%202.0%2Fpackages.png?alt=media&token=6daccbf7-81fa-471b-87a1-47909ba507da - Add the below line in the buildgradle (app) >> sync implementation 'com.android.billingclient:billing:3.0.2' After that, click Clean , Rebuild form the Build menu, and remove the imports manually if needed.