To merge a customer, go to their page in your customer list, and click the ⚙ cog menu in the top right of the screen and select Merge customer:

You'll be asked to search for the customer you want to merge with, and then shown a screen where you choose the details to keep:

The name and addresses will be overwritten, and additional contacts will be merged into the customer you choose to keep.

Only master admin users can merge and delete customers.