From the regular screen while scanning or manually on an object (channel), press PROG, FUNC, F3. This takes you to expert global settings (different than global settings). Repeatedly press the down arrow (17 times I believe) until you get to FlexStep. Then press the left or right arrow keys to turn it on or off. Press F1 (save).
Enabling flexstep lets you to enter a very specific frequency with several decimal places instead of rounding. For example, with flex step off, if you enter 173.21 it becomes 173.2125 by default. Turn it on, and you can enter 173.21000, 173.21125, 173.21250, or just about whatever precise freq you want to several decimal places.
If you use the tune function with flexstep set to on, you will also see many more frequencies to go through than normal.