Confirm that your TI-89 is in radian mode by checking the status line at the bottom of the home screen. If it is in radian mode, you will see "Rad" listed there. If "Rad" is not displayed, change the mode by pressing the "Mode" button and choosing "Radian" from the menu.
Type in the radian value that you wish to convert to degrees. Open the Math menu by pressing "2nd" and then "Mode." Press "9" to select the conversion symbol DD.
Press "Enter" once you're back to the home screen. This will display the precise angle in degrees. For a decimal approximation, press the green diamond button before pressing "Enter."