Wow, this is not a job for the faint of heart, and takes intermediate to advanced skills. I did this back in 1999 before there were iphones and kits to open and fix those. You may read some things like "butter knife" or "exacto blade" in this article and you may have better sputdgel tools of your own. Bottom line is that this is not easy.
I doubled the memory in my HP48G from the built in 32k RAM to 64K of RAM. This was after college because I was too scared to pry open my expensive calculator that I needed to pass my college EE classes. When opening it I scratched the PCB while breaking the plastic posts near the top of the screen, and the traces on the surface of the PCB were broken. I had to put "microwires" to bridge 3 of the traces to "fix" the calculator. I knew those lines had to do with the display because I powered it up and found that three of the scanlines to the display were not working. Thankfully the micro-wires fixed the issue, and I was able to reassemble the calculator and get it back to functional with 2x the memory.
The instructions for doubling the memory are not at the link below. What I recall was that I piggy backed a second 32K chip directly on top of the first one, all connections were identical except the highest memory address. I had to hook that one up with a wire to a different spot (Basically this added a bank of memory and used the existing additional memory address line on the PCB.
https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=125