You could look at some behaviors, a lot of them have a deceleration value.
For example, the bullet behavior (https://www.scirra.com/manual/90/bullet) :
Acceleration (ye I know, it's not called Deceleration <img src="{SMILIES_PATH}/icon_e_biggrin.gif" alt=":D" title="Very Happy">)
The rate of acceleration for the bullet, in pixels per second per second. Zero will keep a constant speed, positive values accelerate, and negative values decelerate until a stop (the object will not go in to reverse).