Can I change a customer's credit balance?

Of course you can!

You can change the remaining credits for any package purchased by the customer.

You can increase the credit balance as long as the count does not go beyond the total number of credits offered. For example, if a customer has purchased a package of 10 credits, you cannot edit the remaining credits to be 11 (out of 10).
You can decrease the credit balance down to 0.

Note: You need to have the Packages > Manage Credits site permission to add or reduce the customer’s package credit balance.

Here's how to go about it.

Go to the customer's page.
Select the Package tab.
In the Package section, select Edit for the package credits that you wish to change.
Enter the desired Remaining Credits amount.
Select the check mark to commit (save) your changes.

Changes made will be recorded in the customer's timeline.