A credit is useful for advertisers only and that is the virtual "currency" of the site. You can top up your balance and spend your credits anything you want. With enough credits on your balance you can upgrade your ads anytime and anywhere you want.
Go to "My Ads" and click/tap on the Clock icon. Note that you are allowed to upgrade only published ads that checked before by a moderator on duty.
Change the timing of your ad
You have to be a confirmed advertiser to do that. If you can't see the "Buy credits" button on your ad's time manage page contact us for further steps.
Buy credits to top up your balance
Just use the "Cancel subscription" button on your ad's time manage page. You will see how many credits you can refund and if that is okay for you just proceed.
Cancelling current plan