To change the animations you should use triggers in the eventsheet.
Triggers like "when character is moving" -> Change animation to "Walk", where Walk is the name of the animation of that object.
Other way is using a behavior, like platform. When you move the player, i.e., you can check in the eventsheet and trigger an event if the object Player with the Platform behavior is "moving", it should be checked in the eventsheet using a condition, and you will change the animation using an action in the right side of the condition.
Don't miss to mirror the object if it go to the left, so, you can save animation frames just by using a single event instead of repeating all of them for the opposite direction.