Follow this guide to change your Twitter username from within the Twitter mobile app
Step 1: Tap the “Me” button in the bottom-right corner of the screen.
Step 2: Tap the Gear icon.
Step 3: Tap “Settings.” Note: On Android, you can access the settings menu from the app’s navigation menu.
Step 4: Tap “Account.”
Step 5: Tap “Username.”
Step 6: Type the new username you’d like to have. If the username is available, a green checkmark will appear next to it. If the username is unavailable, you will need to choose another.
Step 7: Tap “Done” to update your username.