Remove any unwanted apps. Perform a cache clear as well. You can also move files to a cloud storage or PC.
Turn off the Motorola DROID Ultra.
Press and hold both Volume Up and Volume Down keys at the same time.
Press the Power key only until the phone vibrates once, then release only the Power key.
Continue to hold the Volume up and Volume Down keys until the Android System Recovery screen appears.
Press the Volume Down key to highlight wipe cache partition.
Press the Power Key
Make sure Reboot System Now is highlighted and press the Home or Power key to reboot.