To reset the Solo's Pixhawk back to stock parameters you need to install either mission planner for windows or tower for android,
I use mission planner so I'll explain how its done, first download and install mission planner for windows here >>
http://firmware.ardupilot.org/Tools/MissionPlanner/MissionPlanner-latest.msi
For Safety Remove the Props!
And do not install the latest Arducopter Firmware when Prompted!
Once Mission Planner is installed power up your solo and controller and then fire up the mission planner program
(IMPORTANT make sure your PC is on the Solo's WiFi network), when Mission planner is running go to the
Config/Tuning | Planner page and set the Layout dropdown for Advanced,
now you will connect to your solo using UDP once connected you need to navigate to the
Full Parameter List page and search for
SYSID_SW_MREV this parameter will be set at 120 change it to 0 and write the parameters using the write button (on the right of the page).
You have now reset the Pixhawk 2 to stock Solo parameters
Reboot the solo wait till it boots and connects to the controller with blue leds front and red in rear.
You must once again perform both level and compass calibrations on the solo and adjust the sliders in the solo app to your specific values/preferences