You can drag and drop Stories on the individual Epics page to change their priority. Changing a Story's priority in an Epic will also change it's priority on the Stories page.
Priorities in Clubhouse are structured vertically, so the higher up in the table or column the Story is, the higher its priority. To change the priority of Stories in an individual Epics page, first choose the Table view. Then click on the Priority tab at the top of the table to sort the Stories by priority:
You'll notice that when sorted by priority, Stories are first organized by workflow state, then by priority -- this is because you can only drag a Story to change its priority within the same workflow state. You'll also see that stories in a Done workflow state can't be reordered, since they've already been completed.
To reorder a Story in the table, click on the directional icon on the left of the Story entry, then drag and drop the Story where you want it: