Completing R&D objectives. Your engineer will say something like "Okay we need to do some testing this session" or something like that. Talk to him, choose R&D objective then select "Accept" and then run a lap in 1st gear or something cos honestly the lap times to beat are so high you don't even have to try.
What bugs me is that I'm on race 6, I've done 3 of these R&D objectives (dunno if they should appear more often) and I've only unlocked 2 upgrades. I got no "Fast" engine mapping yet.
In Monaco, the engineer told me I had to beat 1:45 so I'd get a "new engine map" (and I have screenshots to prove it), I did it, and I got the 10% brake update.
This game is sadly so bugged it hurts.