You may be experiencing any of these issues:

• Erratic phone performance
• Warnings from requiring payment
• Pop-ups on phone preventing normal use
• Fake anti-virus messages
• Redirection to unintended website or app downloads
• Unaccounted phone bill charges
• Friends receiving unintended SMS or emails
• Unintended posts or upload to social media sites
• Appearance of new apps
• Unexpected data usage
• Noticeable reduction in battery life after the installation of the new app

A preventive measure is to place your phone in Safe Mode. Safe Mode prevents third-party applications from functioning and is useful when troubleshooting issues caused by installed apps or services. See How to boot into Safe Mode. 

Follow the steps below to identify the source of the problem and take appropriate masures:

1. Press and hold the POWER button until the Phone options menu appears.
2. Tap and hold Power off until the Reboot to safe mode prompt appears.
3. Tap RESTART.
4. Go to Settings > Apps and look for any suspicious or unfamiliar Downloaded apps. Tap the app, and then tap Uninstall or Disable.
   If you see the fake “Adobe Flash Player“, it will most likely not allow you to uninstall it but may uninstall itself if you enter a random, long-enough number to satisfy the MoneyPak (15-digits) demand.
   If you do not see any suspicious apps, uninstall any recently downloaded apps that were installed shortly before the symptoms appeared.
5. Go to Settings > Security > Device Administrators to remove apps which do not provide the option to uninstall or disable.
6. Uncheck the app to be uninstalled and Deactivate it.
7. Uninstall the app.
8. Reboot your phone.

A factory data reset can also resolve some software issues, and will remove any malicious software. See How to factory reset a device.