RFID RC522 Serial Communication converting problem


I am new to the RFID Scene and Serial communication as well.

So I am trying to use the RC522 RFID module and I wrote in block 22 - 0x36 which is 6. I want to write a code where I take whatever is in block 22 - 0x36, convert it into a float - 6.00 and then do some addition or deduction for example 6.00-2.5 = 3.5 and then I want to rewrite the value of 3.5 into that same sector but of course, convert it back to HEX ASCII - 33 2e 35. I've tried quite hard to find the solution online but couldn't. So is this possible to do?