The workout menu on your watch

Press BACK on the set overview screen and the workout menu opens — the way to fix a set, jump around the workout, finish early, or throw the session away. On the cardio screen, MENU or BACK opens a cardio version of the same menu.

What this menu offers

Resume — closes the menu and carries on where you were.
Undo last set — takes back the most recently logged set and lands you on the set overview for it, with the weight and reps you'd entered still in place: adjust and log it again to fix a mistake. The row is always there so the menu never shifts under your thumb; before anything has been logged it reads nothing to undo and does nothing.
Jump to exercise — the list of every exercise that still has sets left, the current one marked current and the others by how many sets remain. Finished exercises aren't listed, and once everything is done the list just says Nothing left. Pick one and it becomes the current exercise — the way to grab a free machine now and come back to an occupied one later. A superset moves as one block. Holding UP (MENU) on the set overview opens this list directly.
Finish & Save — saves the workout as it stands, shows the summary, and closes the app from there. Sets you never got to are simply not logged; the app never fills them in for you.
Discard workout — abandons the session without logging anything, on the phone as well as the watch. It can't be undone, so it sits last.

The cardio version

The cardio screen's menu is the same idea with the cardio-specific rows added: Resume, Finish this cardio (ends this exercise and moves to the next one — or to the summary if it was the last), Next segment (interval cardio only, the same as pressing DOWN), then Jump to exercise, Finish & Save and Discard workout as above.

Resuming and leaving

If a workout is already running on your phone, the watch may ask "Resume …?" on its own the next time it syncs — Yes joins it wherever the phone is. Saying no doesn't lose anything: Resume workout stays at the top of the Start Workout menu, and the watch won't ask again for that session.

Leaving the app mid-workout — pressing BACK out of the Start Workout menu, or a long press of LIGHT — pauses the watch's live syncing but doesn't end the workout: it's still on your phone, and still offered under Resume workout on the watch. The one thing that doesn't survive is an outdoor cardio GPS recording, which stops when the app closes and can't be reattached, so finish the bout before you leave.