OK, this might might be a necro, but man, this needs to be answered:

So, when you open the schedule you'll find that the hour bar at the bottom is all yellow, meaning that all hours are selected for modification. Would be better if, by default, none were, but that's the lot in life we've been given.

What you need to do, is de-select the hours you don't want to edit. Easiest is to shift-leftclick your desired hour to edit and then expand your selection with simple left-clicks.
Then just enter the desired priorities and you're good to go.

Hope I could help :)

Might be a necro as well, but this helped me a LOT, i was actually unable to figure this out until i found this explanation.

this helped me as well, thanks

I've found it gets cut off with lots of colonists. My current fix is using the Misc Robotos Work Tab Patch to let me switch to them. Pick a schedule, then switch back to scheduling colonists.

@catch lightning I can confirm it still gets cut off with lots of colonists.