In the setup screen, once you enter the server (or an offline race) It's the wrench icon where you make setup adjustments.. The last entry is pit strategy. You can do 3 different scenarios. But you can only change "tire type, fuel amount, air pressure" All other setup adjustments will remain the same. Don't think there's a way to change those once a race starts.
I've got my #1 slot set to medium tires, 87 litres of fuel, same psi as Im starting the race. (If all goes well, I should be able to get to half way and that will get me home)

#2 I have set to soft tires, 45 litres fuel and +1 of pressure to each tire. (in case I need to pit later in the race and want to try softer tires)

#3 I have set to super soft, 20 litres, + another pound added to tire pressures. (In case my race is toast and want to try turning a few hot laps on super softs)
Once you have your setup and pit strategy set, save/overwrite the setup. Then once in the race, when you enter pit road you'll see the pit strategy box pop up. Use the arrow keys to pick 1, 2 or 3 (First stop it should use #1 by default) but -if- you want to use #2 or #3, or make changes to the scheduled stop, use the arrow left/right/up/down keys to make adjustments. You can also change stuff from the scheduled stop -while- you're coming down pit road, but that'll take some coordination and probably should only be attempted if you've messed up somehow and want to try something different on the fly. (for example you want to run hard tires and 100+ litres of fuel) You can change what's "scheduled" but you have to do it before you get in your pit box. Once in the pit box, the crew does whatever is scheduled on the pit strategy screen.
* Make sure and get all that down.. There will be a pop quiz tomorrow morning!