How to re-program Prusa firmware to accept a taller Z axis?
A .hex file is of no use to you, because it consists of compiled firmware which is very difficult to edit. You need to go to Prusa's GitHub and download the source code. Then, find the header file for your model of printer, and change Z_max_pos to the correct value. Finally, you need to compile and upload the firmware to your printer following the build instructions (see README.md). You will need to have the Arduino IDE installed to do this.