Didn't upgrade the drive but used these instructions to open up the device and clean the fan.

There are two spring loaded screws which hold the assembly to the case. These are visible after you remove the drive. Getting them back is a little tricky.

I removed the reset switch assembly from the case along with the power connector instead of disconnecting it from the PCB

My PS3 did not have the convenient cutout as in the picture of step 17. Instead there was a grill through which I could see the battery. I had to figure out how to remove that plate completely.

You have to disconnect the fan power, and the Bluetooth and WiFi cables from the board, then open around 5 marked Philips screws. Unscrew 2 smaller screws holding the hard drive assembly to the case.

Then you have to lift the motherboard from the case by pulling at the other end of the board from the fan. Underneath there are another set of marked Philips screws. Two sets of larger Philips screws hold down the brackets for I guess the CPU and GPU. Finally you can get at the coin cell and replace it.

Absolutely ridiculous design, probably meant to make the consumer run to a Sony service center. Unfortunately I did not take pictures of the process and hopefully the battery will last another 5 years or so before I have to do this exercise again.

One more thing. The Blu-Ray drive in my unit has different cabling. One of the cables is underneath the drive and might easily get damaged. Replacing it back in the socket also requires care and time.