Re-applying thermal paste is sometimes very necessary to prevent your laptop from overheating, which can lead to damaging the laptop. Depending on the thermal paste and the type of laptop user you are, thermal paste dries up between 2 to 5yrs of using the laptop.
Yes, repasting a laptop could void the warranty because before you apply a thermal paste, you have to open up the laptop to get access to the heatsink, the CPU, and the GPU in order to re-apply the thermal paste, and most manufacturers void warranty if you open up the laptop.
No need to worry about this because most of the time, before you see signs that your laptop needs repasting, your warranty might have expired already, however, if your laptop needs repasting within the warranty period, I suggest you take it to the manufacturer’s service center, they will re-apply the thermal paste for you without voiding the warranty, but it will cost your extra cash.
If you have a way of opening the laptop without breaking any seal or sticker, the manufacturer won’t be able to tell if you have opened the laptop up or not.
Also, not all overheating means your laptop need re-pasting, sometimes you have to check for dust clog because if dust clogs the vent, it will prevent airflow which can also lead to overheating, so always make sure you clean dust off your laptop.
Again, don’t push your laptop as we do with desktop PC, desktop PC has good cooling solution and also better airflow because of the large cases and more vents. You can play games on a desktop PC for 24hrs straight, but not all laptops can even do 8hrs straight gaming. It will overheat.
So, know how to use your laptop, know when it hits the limit, and make sure you are not blocking the vents on the side and most importantly the bottom. These bottom vents can easily be blocked by placing the laptops on the bed whiles using them.
The warranty is void if you open up your laptop, and you need to open up the laptop before you can re-apply thermal paste, so yes, repasting a laptop voids the warranty. You could also call the manufacturer’s service line for help.