Or add velocity envelope, right click & "Add Point" (you'll want 2 close together at the start & 2 close together at the end) & then drag the line between the two inner points down to the speed you want as a percentage of the original.

(the larger the gap between points 1 & 2 or 3 & 4 , the longer transition from original speed to target speed)

Or use Velocity Wizard in Excalibur which lets you set %markers at the precise point in the footage you want to make changes. It will also resize the event in the timeline to match the changes you've made.

Gary