Edit the callout in the Dimension PropertyManager.
You can specify precision, select an arrow style, or add text. However, you should retain the dimensions and symbols for the size and type of hole.
If the hole has been created by the Hole Wizard, you can also click Variables in the Dimension PropertyManager to access a list of Callout Variables to insert into the hole callout.
If you attach a hole callout to a tapped hole in ANSI inch standard and the current drawing units are millimeters, the drill diameter and hole depths are reported in mm but the thread description retains the ANSI inch size designation. You might want to replace the thread description with other variables from the Callout Variables dialog box.