If you notice you Microflow carefully you are changing the input param Order but you are committing another object(s) or list int he commit activity in the end.
So you can assume: the Input Param Order and the OrderList though are theoritically same data BUT they are separate instances. So you are changing 1 instance but committing another which obviously does nothing.
So you have to change the correct instance and commit the correct instance.
Remove the OrderList getting and orderlist commit actions. In your existing Change action on Order object, choose the options Commit to YES and refresh in client to YES. That should do it.
If you want to the change the status of all this orders then you would need a for loop.
In this for loop change for each order in orderlist the status to completed.
A for loop is the blue activity on the top. It literater over each object in your list.
If you want to change the status of only one order (your yellow parameter in your microflow) then just change the status of the one order.
You can do it with a change object activity.
Hope this helps,