In order to change order customer, you need to have Magento 2 Convert Guest to Customer Module installed on your store. If you already have it, please do next:
1. Navigate to Sales > Orders and choose the order you want to be assigned to another customer.
2. Then you have to simply press the Change Customer button in the Account Information section.
3. Then there will appear a grid with all customers. All you have to do is choose the customer you want to assign to the order to and press the Assign to Order link.
4. So that's it, you can see that the order customer has been successfully changed.