insert two tempo markers, one at beginning and one at the end of the section you want to slow down.
double click the first marker (or right click and select edit). Set this marker to a slower bpm and the rest of the song will be slow until it hits the end marker, which should be at your initial bpm
If you want an immediate slowdown, make the sure the marker is on 'hold'. If you want it to gradually slow down you have other options like smooth/fast/slow/etc
ALSO, and this is key. Make sure in clip properties, you set the type from 'efficient' to 'pro', this means the computer will put more processing power into the slowdown and it won't sound as choppy.
hth, if you still can't get it to work lemme know