When you click on the instance to pick it then all instances are narrowed to just that 1, so you should first store the UID of the one you picked as an action probably in a local variable.
Then in a sub event of the click event, open up the instances again by using Pick all instances, then you can set the new animation on all instances. For the one you clicked earlier you can then pick instance by UID and set the correct animation since it would've just changed with the pick all.
You could also set a variable to a different value on the one that was picked, you just need a way of differentiating it from the other instances.