Simply complete the third year of Season Play.
To do this you need to finish the last race in the World Championship in your third year. If you want to make progress towards your level 50 achievement I suggest you race all your races. If you wanna finish year 3 as quick as possible you should pick all the short races (the ones you earn the least credits for) and if you want to make it easy, put on all the assistance you can.
If you really don't feel like driving yourself anymore, you could always hire a drive. If you do though, you won't get any experience for your level 50 achievement.