I need to renew an add, but I cant see how or where to do it. Of course I can just wait till it expires, then do a new post, but there must be an easier way.

From FAQ. On the site https://classifieds.thaivisa.com/

:How long does my ad stay on site? 
"Ads stay on site for 30 days.  Three days prior to your ad expiring, you will receive an e-mail reminder.  You will have an option to RENEW your ad for next 30 days.  You just need to be logged in and simply click on the appropriate link.
If you would like to EDIT or DELETE your ad prior to the expiration date, please see further instructions in the list."