From the Seller Hub here:
https://www.ebay.co.uk/sh/ovw
Then go to My eBay >> Selling >> Active Listing.
To the left of the item(s) click on the down arrow beside Edit, and from the drop down menu if the item is eligible , you'll see the link to ' Promote'.
The above is when using a PC / laptop I don't use the app where the instructions may be different.
@resselers