If you have the Android SDK installed on your computer (otherwise, see e.g. Is there a minimal installation of ADB?), you can use the
fastboot utility for that xe2x80x93 provided USB Debugging was activated on the device (which might be the default in recovery mode):
fastboot erase data fastboot erase cache
These two commands should erase everything from the
/data (user-apps and data) plus
/cache (app cache) partitions. So then you only had to take care for information stored on the SDCard(s), which you could erase e.g. via UMS/MTP (concerning the internal SD). There might be ways to do this via
fastboot as well.
Take care to not erase things like system or recovery, as that would render the Android system unusable xe2x80x93 which is not what you want. Entering the
fastboot command without parameters lists up what you can do with this utility (syntax etc.).
For more details, you might want to watch some introductionary video: Fastboot clean erase tutorial: that describes a complete erase of the device, followed by a re-flash.