To use this option, simply log in to your account and click on the renew button. If the button is grayed out and it says 2 soon to renew, then you should try it at a later time. In general, ads can be renewed once in every 48 hours.
For guest members, please check the email you have received from us. There is a link available for this task. Simply visit the link and click on the renew the ad option. Once again, if it was not clickable, then you have to make sure the 48 hours time has passed since you have posted the ad.