I guess you mean this one
https://www.mobile-solarpower.com/the-minimalist-great-for-small-vans-and-cars.html, confirm?
Assuming you followed Wills instructions to the letter...
Preparation:
Get another 12 volt battery same as the original.
Get a new 24 volt inverter of up to 2000 watts to replace the original.
Make sure the existing battery is fully charged.
Fully charge the new battery.
Parallel the batteries to equalise them(they should be within ~0.2 volts before you parralel them).
Put the parralel batteries in your system and use the system for a week to make sure they are well balanced.
Upgrade:
Disconnect your panels and any other charge sources.
Disconnect the inverter from the system.
Disconnect the batteries from the system.
Replace the 12 volt inverter with the 24 volt inverter.
Connect the batteries in serial to make a 24 volt battery.
Reconnect the batteries.
Test the inverter to make sure it can power a load off the 24 volt battery.
Make sure your solar charge controller is configured for 24 volts if it doesn't automatically configure itself for 24 volts.
Reconnect your panels and any other charge sources.
UPDATE: not sure how your panels are configured.
They may have to be connected in serial to make the correct voltage.
Hopefully someone else here can speak to that point.
