If it is on an iphone go through safari and sign into ebay and then at the bottom of the page click  on classic site and then it works the same as your PC.

If you have the ebay app on your phone, you select selling from the home page - press the box - where your number of sales are - then select unsold, then the item you wish to relist.  There is an option to relist, and an option to sell similar.