QuickBooks Online offers a user-friendly platform that enables you to manage your customers with ease. It's essential you're able to move customers from one email access to another. Don't worry, I'll be glad to provide the information you need to move forward with your work and get back to doing what you love.
Based on the details you've provide, I suspect you're looking to change a customer's email address. If that's the case, you can do this by clicking Sales from the left menu > Customers > locate the customer in question and click on their name > hit Edit and make the necessary adjustments. Make sure your info saves once you're done.
In case you're referring to QuickBooks Online Accountant, I encourage you to check out the helpful resources below: