Related List are meant to provide you an easy way to identify the existing relationships in your data, it is only logical that you should be able to organize them in a way that makes sense for you.
Order Related Lists
To get started, click on your profile picture and then on 'Settings' (1). On the left-hand side menu click on the option called 'Related Lists' (2).
First, select the corresponding tab (1), to view all the existing Related Lists. Once there, changing the order of your Related lists is quite easy. Just select the element that you want to move up/down and click on the corresponding button (2) as many times as needed until it reaches the desired spot on the list.
The next time you click the 'Things You Can Do' button, or the dropdown available next to the 'Activity Timeline' section within the corresponding tab you will see your Related List's new order.