To disassemble laptop asus FX504, you need to unscrew 6 screws on the bottom cover of laptop
Use a plastic tool, to open laptop case.
In description of this video, you can find useful links to various laptop disassembly
tools and parts.
After removing bottom cover, disconnect battery connector from
the motherboard.
Unscrew 6 screws that hold the heatsink.
And lift up thermal module.
Remove old thermal grease from CPU and GPU.
Also remove thermal grease from thermal module.
If thermal interface of video chips in an elastic state, its replacement is not necessary.
To replace thermal paste I use Noctua NT-H1.
In the description, I left link where you can buy thermal paste.
Apply a small amount of thermal paste
to CPU and GPU.
Install heatsink and screw 6 screws.
Also you can clean the fan from dust.
Connect the battery connector to the motherboard.
Install bottom case of laptop and screw all screws in the order show on this video guide.