If you have purchased credits or had your subscription renew with new credits, you can apply them to any free event you have posted. Look for the "Upgrade to Premium" green button on the event in the "My Events" section. You can find your events when logged in at app.evvnt.com/events.
When you click to upgrade, you will be taken to a checkout page. If your credits are applicable to your event, then you will be able to apply your existing credits. If you are asked to pay again, then it may be that one of your event categories does not fall within the price plan for your purchased credits. More info here: Q: I already paid so why is it asking me to pay to upgrade my event?