To merge a customer, head to the Customers tab at the top of the page, select the customer you wish to merge, and click the ⋮ menu in the top right of the screen. Select Merge customer:
You'll be asked to search for the customer you want to merge with, and then shown a screen where you can choose which details you would like to keep/replace.
The name and addresses will be overwritten, and additional contacts will be merged into the customer you choose to keep.
Please note that only master admin users can merge and delete customers.