Here you can see all your icon sets that are currently on sale on Iconfinder. Click on the name of the icon set you would like to add new icons to.
And finally, once you are seeing the icons from the chosen icon set, click on the green button called "Upload Icons", as in the screenshot below:
Next page is the normal upload page. Follow the instructions provided and you are good to go. In case you need more help, check out the How to upload icons to Iconfinder guide.