This is the wiki page for the console command:
This command allows you to increase the value of a cvar by a given numerical value.
To use the command, open the console and type:
cvar_add cvar_name 5
Assuming the value of cvar_name was 3 when you run the command, the new value of cvar_name would be 8.
Real World example:
cvar_add sv_soccar_gamespeed 0.1
Running this in the console will increase the gamespeed by 10% of default speed.
Tip: You can use the bind command or the GUI to bind this entire statement to a keyboard/mouse/controller key. This way, you can increase the game speed in small steps every time you press the button. If you don't have a free bind, you can use the alias command to reduce the amount of typing required.