On your dashboard, go to the Training Plan tab and simply Left-Click (under the workout/in the white space) and hold your mouse/touchpad on the first day you'd like to mark as such . Then, drag the cursor across the other days you'd like to take off.
This will select those days and you'll get the confirmation dialog. "Mark as time off" will be automatically selected. If that's what you want to do, then simply click "Save." If you want to change to an indoor workout, click that option and "save".