Select Manage products from the Contributors drop-down menu as pictured below.
From the list of the packs, choose the pack that you want to add items to, respectively. Then, click on More and choose Add items to pack.
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 this guide on How to upload content to Iconfinder.
Replace an item
Select a pack from the Manage products page. Click on the three dots next to the item you want to replace. And choose Replace item. Find the item file and confirm.
Delete items from a pack
Select Manage products from the designer drop-down menu.
Find the pack from which you'd like to delete items and click on More and from the drop-down menu choose Tags.
Find the items that you want to delete and click on the X button to delete the item from the pack.
If you change your mind, you can restore the item at any time.
Read more about How to manage packs and families.