1. Select an annotation you want to change, select Style to open up the annotation style dialog.

2. You can change the style of an annotation quickly by tapping on a preset from annotation style picker.

3. You can then change the style attributes of your presets (i.e. color, stroke, opacity etc.) by selecting them in annotation style picker:

4. You can also modify your annotation's style without changing your selected preset, by tapping on the selected preset again to deselect it.