Alright, you had me curious, so I went out and messed with my DRO for a minute. Here's the secrect:
1- Turn the DRO off (or start with it off).
2- Turn the DRO on.
3- When it starts doing its count up to 9, press the 'S' key (to the left of the model number, very lower LH corner). This will enter a configuration mode.
4- Press the up/down arrows until you see 'INCH DOT'.
5-Each axis has the number of decimal digits in it. I THINK you press the 'x' 'y' and 'z' buttons to cycle through the values for each.
6- Press the 'S' key again to leave config mode.
Note there are other useful things in this config, such as scale type (1um vs 5 um, etc), AND + direction.