How to replace the thermal paste of the cpu and the type of it?

I'm not sure if its time to re-paste your CPU and given your first attempt in repairing a similar MacBook Pro is it wise to tempt fate?

I would instead focus on the simpler tasks like cleaning the dust build up within the fan blades and heat sink fins. I would install a good thermal monitoring app like TG Pro so you can monitor your systems thermals and fans. I would also monitor your batteries health as well using CoconutBattery as well as oftentimes the battery needs to be replaced before the need to re-paste the CPU.

Re-pasting the CPU is a sizable job as you need to fully remove the logic board and the screws which hold the heat sink on are well sealed, so it will take a bit of effort to unseal them either with heat or solvent.

One of the biggest issue I encounter is many people over do the TIM! A little goes a long, long way! There are two ways for you to visualize the correct amount. For starters get some facial tissue, pul the ply's apart so you only have one thin sheet now cut it into the size of the chip, This one is a bit hard as you hav three within the carrier as you can see here

Now wet it and roll it into a tight ball, note the size, even this is a bit much.

Find a small coin and using some toothpaste emulate applying some toothpaste on the coin of the same scale we just emulated and then pressing it down on a clean flat plate, Did the paste squeeze out? If it did it was too much!

Here's an example of too much! Even Apples assemblers do too much!

So why worry about over doing it? Many of todays TIM paste interfere with the electrical signals of the capacitors which dot the carrier. These SMD devices are so small it doesn't take much to mess up their values. As an example a good meter or scope can see your touch across these small devices. If your finger was wet the moisture and the salts within your sweat can even alter the circuit! Which is why many of us wear gloves when working on sensitive electronics.

As far as what TIM I would stick with either [linked product missing or disabled: IF179-010] best for most people or Thermal Grizzly - Kryonaut Extreme if you are a video pro doing heavy video/image rendering.

As you have a broken system already practice on taking it apart if you are still going to re-paste the CPU and take note which chips have TIM. I personally haven't done this model so I can't tell you if one or two chips might not have any TIM on them! This is by design, don't over think it. Think this in your head: Monkey see Monkey do! Without the full understanding we shouldn't second guess the designer!