If you already have the animation for walking in one direction, then you could use an event like this:
(Condition)-Keyboard: Right Key is down (Action)-Player Set animation to "Walk"
Then, to make the player turn around, use this event:
(Condition)-Keyboard: Left Key is down (Action)-Player Set animation to "Walk"
(Action)-Player Set mirrored
To make the animations stop playing when you stop pressing the keys, use these events
(Condition)-Keyboard: On Right key released (Action)-Player Set animation to "Stand"
(Condition)-Keyboard: On Left Key released (Action)-Player Set animation to "Stand"
That should do it. Let me know if it works!