Replacing a gas control valve is not complicate and it doesn't require any special tools, but we always recommend calling the licensed technician for any complex and sensitive service work. Below are the instructions and link to Bosch website with more details and pictures.
Tools: Phillips and small head screwdriver, 4mm Allen wrench, adjustable wrench, combustion gas analyzer.
Turn off the water heater and main gas shut off valve that supplies the unit.
Remove two Phillips head screws that hold the front cover in place.
Take off the front cover by pulling it away from the heater and upward.
Unclip two upper and lower clips to remove the combustion chamber cover.
Take out three Phillips screws on control unit, ensure the engaging tabs are positioned and lower the control unit.
Locate the gas valve and unplug the wire connection and silicone tube.
Disconnect the large hex nut.
Remove the three Allen screws located on the top, and that will free the gas valve.
When installing the new gas valve it is important to check and replace if necessary the fiber washer found on the left and rubber gasket on the top of the part.
Put back the three Allen screws and the hex nut.
Open the manual shut off gas valve.
Once the gas valve has been installed, check for leaks by using the soap foam. If any bubble appears, the gas valve leaks and you should fix the connections.
Put the silicon tube back and relink the wire connection.
Put back the combustion cover, control unit and the front panel.
Resume the power.
If the unit runs properly it is important to calibrate the unit and if it doesn't than you must adjust the gas valve.
Detailed guide here.