If your phone gets slow or doesn't work as it usually does, it may help to restore the factory default settings. All settings and configurations you've made on your phone are deleted.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Reset options.
Press Erase all data (factory reset).
Press Reset phone.
Press Erase everything. Wait a moment while the factory default settings are restored. Follow the instructions on the screen to set up your phone and prepare it for use.