To change your signature in the Android version and remove the Boxer tagline:
1. Tap on the button in the top right corner of the inbox. If you don't see this button, use the hardware menu button most likely located on the bottom left corner of your phone.
2. Select "Settings".
3. Select your account that you would like to change the signature for.
4. Tap on "Signature".
4. Type in your new signature and select "OK" to save it.