Open the software to check the problem. If it is a bug in the software itself, you need to help customers solve it.
If it's not the software itself, you need to look at the terms in the software contract. If it is free, then change the software for the customer.
If you need to charge, then choose to discuss the price and payment method with the customer first.
After the customer agrees, start to modify the software.
Complete the software modification and submit it to the customer.