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Sorry to hear you’re having trouble.
RPN means reverse polish notation, a different type of entry syntax often preferred by engineers and physicists as it can be quicker.
Without knowing the model of your calculator, I can't give specifics.
If you look on the keypad though, not at the actual keys but above them, you should see a line that says RPN. Usually pressing the 2nd or Fn key + that RPN key will toggle it on/off.
For most HP models, you'd press the MODE button first, then the desired mode such as 4 for ALG (algebra), 5 for RPN, etc, following the outlines above your keys.