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.

Magento 2 Convert Guest to Customer  Configuration

2. Then you have to simply press the Change Customer button in the Account Information section.

Magento 2 Change Order Customer

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.

Magento 2 Assign new customer to the order

4. So that's it, you can see that the order customer has been successfully changed.

Magento 2 re-assign customer to the order