YRT describes the detour as
Southbound: left on Denison, right on Old Kennedy, right on Steeles past Kennedy to regular routing.
(Note: I've updated this with the new information YRT posted on their website.)
Keep Cool on the Bus
By the way, have you noticed how hot it's getting outside? The Region has issued a heat advisory warning on account of the high temperatures and high humidity Environment Canada is expecting for the area between now and Thursday. In addition to the Region's own tips on keeping cool, if you need to be out running errands today consider making the trip by transit instead of walking or biking—all of YRT's vehicles are air-conditioned and can help you stay out of the sun and heat.