[ChuckF.]

Raise the door all the way up. Put a C-clamp in each side rail, blocking the path so the door can't go down. Now the springs should be relaxed enough that you can unhook them. If not, mark the position of the threaded rods that adjusts them with spray paint, and then loosen them until you can unhook the springs. Tie the springs up to the rails so they don't flop down.

Unhook the garage door opener from the door using the disconnect device. Now the springs are no longer holding up the door, only the C-clamps are. Put two more C-clamps at a lower position at about halfway point. With someone there to hold up the weight of the door, release the higher C-clamps and lower the door to the second set of C-clamps.

Remove the first set of C-clamps and move them to just below the bottom rollers on the second panel. Remove second set of C-clamps to free the bottom panel.