You should be able to change the refresh priority for individual extracts. -- Which is particularly useful for extracts that are on the same schedule.
Like Siva, I am no longer seeing this info. Or any priority info on the Tasks screens. Has something changed with an update?
I am also having this issue. I cannot see or set priority for schedules in Tableau Online.
We were using this fairly heavily in order to make sure our data sources were properly synced. Please let us know if this is an intended feature removal or if this is a bug.