What version do you work with?
the question is a little generic. .
Do you want to slow down the whole video evenly or just at some points?
there is the chart to vary speeds in the various steps if you are working for keyframe.
if you go to the timeline you can activate the movement chart, and manage it as you want, otherwise if it comes to slowing everything evenly you can simply select all the keyframes and move the cursor to the extremes of the selection by dragging it right. .
hi clamidosaur, to do this you have to increase the number of frames, so doing animation will have the duration you prefer and below the movements will be slower.