To switch the HP20b calculator in RPN mode press [Shift][Mode]. Press [Up Arrow] or [Down Arrow] repeatedly to scrollthrough the menu till you see " Chain" or " Algebraic" item is displayed, press[INPUT] key to cycle through till you see “RPN”. To exit the Mode menu, press [ON/CE] key once.

RPN is short for Reverse Polish Notation.

In RPN mode, numbers are entered first, separated by pressing [INPUT], followed by an operation key.

Note: pressing [INPUT] is optional after entering a number if the next key pressed is an operation. Each time you press an operation or function key, the answer is calculated immediately and displayed.

For example, suppose you wanted to add two numbers in RPN. 1 and 2. Press [1] [INPUT] [2] [+]. The result, 3.00, is calculated and displayed immediately on the bottom line along with the (+) symbol on the top line.

Read more about RPN from this link.