Grid Assist can be enabled in the Race Setup screen before starting your championship season.

When doing a race, you must select Qualify. You will then be given the option to End Qualifying, after that you can select the Edit Grid option. You can change the grid however you please from that screen.

As a sidenote, if you quit a race the results at the time of quitting will be the final results for the race. This means you can effectively choose the entire race standings by editing the grid and then retiring immediately. This can be used to prevent championship rivals from getting points.