Victoria 2 has a lot of commands, but unfortunately there aren't any commands to directly increase or decrease infamy. Thankfully, there are events that you can run with commands that will increase and decrease your infamy pretty quickly!
To use the event command, you'll need to know how to open the console. To open the console in Victoria 2, press the ` (grave) or ~ (tilde) key on your keyboard. If that isn't working, or you need more help, see this guide.
The event used to decrease infamy is the "Liberty Prevails" event (event ID 18540). This event gives you the option of losing either 2 infamy and gaining prestige, or losing 3 infamy.
To run the Liberty Prevails event with commands, simply type the following command into your console:
Run the above command as many times as you like, until your infamy is at the number you want.
Infamy is increased with the "Heart of Darkness" event, which has an event ID of 14670. This event will give you the option of gaining 5 infamy and losing 5 prestige. To run this event, type the following command into your console:
Repeat the above command until your infamy is where you want it to be.