Under the top edge of the track-pad is the memory. Top right can also be storage. The heat sink is at the top section of the keyboard. Since you replaced the battery, I will assume you know how to take apart the laptop. To change the thermal paste, you need to remove the heat sink. This part is the large black part in the center of the board. The best way to see whats getting hot is by opening the laptop in a L shape form, laying the laptop on screen with the bottom facing you and feeling different areas for heat but this is dangerous because you can damage the laptop if you touch something you shouldn’t. But you can change the paste first and see if that makes a difference. Just make sure you disconnect the battery before doing so.