.2. By pressing the TOGA switches a second time.
Smokey,
On the take-off roll, above 80kts, all that does is disarm LNAV and VNAV- you need to advance the thrust levers manually to get any more grunt.
King,
Can't answer your question as I've never flown an A/C that HAD T/O derate- but, would cancelling it through the FMC work?