Solved

Upgrade car

The first races of the competition I finished in the top. But the last races I finish around position 6 till 10. What am I doing wrong?

And the points I get after the race where should I spend them for? For example only on acceleration or handling. Each race I earn 3 points

Handling, Downforce & Fuel. Running Bull
Acceleration, Handling & Downforce. Murk
Acceleration, Braking & Handling. Will I am
Acceleration, Braking & Downforce. Tifosi
Handling, Braking & Downforce. Wonder
Braking, Handling & Fuel. Rednote
Reliability, Handling & Downforce. Toymotor
Cooling, Reliability & Tyres. Cosurworthit

Slap them on where you are legging behind the most.

Where did you get this list? Because I didn't know about it and didn't find it on the internet?

Make sure to read the Rules and Regulations accessed through the "?" at the bottom of the main dropdown menu. At the Cars page look at the review screen. You will receive additional upgrade points after each race as seen in your mailbox after the race. Only leave green checkmarks in the areas that you feel are most crucial to developing your car. That is where those additional points will go. You will get more points in the areas where there is more distance between your cars last race and the most developed car in that area. This usually amounts to more points than you are given by your headquarters. Apply your headquarters points, three points at the moment, to the areas that you think will help your car the most.
Acceleration, braking, handling and downforce are most important but you may get more development in those areas from the green checkmarks so you might want to put your headquarters points in some other area.

James made up that list as an illustration.

7 teams? looks like i need 2 more to compete james

No lists, made it up to help you pick a design route or get you thinking about a combination you might like to try

Plenty of names a buddy could pick lol
Baby Bull. Handling, Cooling & Braking
Indian Army. Acceleration, Cooling & Reliability
Gas. Fuel, Braking & Tyres
Mentor. Fuel, Acceleration & Cooling

I'veel upgrated myrthe headquater to get more pointer.

Should I use all the checkmarks or just the things where I'm far behind? For now I just use one or two. Does the staff have to do anything ?

First things first get all ratings a gray colour then you can leave out Cooling, Reliability, Tyres & Fuel.
The rest you want green and maxed out before working on them others above again.

Should I start with one item then? Because now they are all red. Because the are between 9 and 13

Or did you mean the checkmarks?

On the overview tab try to get acceleration, braking, handling, and downforce to turn gray first by adding points to those areas. On the review page there is a percentage number at the top that goes up as you checkmark fewer areas. That means that you will get a higher percentage of points in those few areas you have chosen than if you chose every area at a lower percentage.

2 races left & car ratings are around 9 & 13

Do one at a time, biggest gap first, if it's the weakness it will take 3-4 goes to break level, anything else should be lots quicker

You need 4.5 or 5 star CD & TD to get lots and lots of points in one go.