Sometimes it's helpful to login as a customer to help them update their information. It's super easy to accomplish this!
First step is to login to the merchant portal and navigate to the customer details page for the customer that you'd like to login as.
On the right side of the page, you'll see a large "ACTIONS" button. Click this and you'll open up options that include 'login as customer'. Once you click to login you will be taken to the customer account page for the customer you specified.