To edit your listing on SidelineSwap:
From desktop web: Click the three dots on the item thumbnail and then "Edit" from the menu. You can also click to "Edit" the listing from the item page itself.
From mobile web: Click the pencil icon below the listing from the locker view or the pencil above the listing from the item view
From the App: Click "Edit" on the item in your locker or, from the item page itself, click "More" from the action menu and then "Edit Item".