My customer's currency doesn't appear in their customer info. So, I'm not able to edit my customer's currency.

Hi there, 

Glad to have you connect with us here. It's important you're able to manage your customers with ease in QuickBooks. Don't worry, I'll be happy to provide more info so you can get the resolution you need. 

In order to assign a foreign currency to a customer you'd need to be using the Essentials or the Plus subscription plan. With the Plus or the Essentials, you have the flexibility to turn on Multi-currency that then allows you to assign the foreign currency to the customer. Here's a helpful article that provides info on how to set up and use Multi-currency.  I also encourage you to check out the following article with more info on how to: Assign a foreign currency to a customer in QuickBooks Online.

Let me know if this info helps. I look forward to hearing back from you. 

I already have the multi currency option turned on. I can’t change currency on an existing customer but I can add different currencies for when I create a new customer. 

Are you looking to add a new customer with the correct currency then merge that customer with the existing one?