Enter a number, then hit the units menu and select the unit associated with that number. It may be N/cm^2, or something, it doesn't need to be just a single unit. Then use the arrow looking button, ->, that is next to the green shift key. It is the 2nd of the MODE key. Then enter the second unit you wish to convert to.
Example:
5_N/_cm^2 -> _lbf/_m^2
Output: 11240.4 _lbf/_m^2
<--- Loves his TI89