How can I raise my spending limit in Apple Pay?

If you are asking about your Apple Card credit card, each time you make a payment it automatically increases your available credit. The available credit (amount you can spend) is the difference between your credit limit and the total amount you currently owe. If you want to increase your credit limit, see: See instructions in: View your available Apple Card credit details or request a credit limit increase - Apple Support

Each time you make a payment it automatically increases your available credit. The available credit (amount you can spend) is the difference between your credit limit and the total amount you currently owe.

See the following from How to make Apple Card payments - Apple Support

Note the "Available" amount in the Card Balance section.