From anywhere within your library, simply click on an item to update any details associated with that item.
If you are on a board, click any board item and a sidebar will appear for you to then edit the item's details.
Please note, any changes made will be reflected anywhere else you've used that same item within the same project. Here's why! What a bummer it would be if you updated a price increase on one board within a project, yet forgot to update on another. See how confusing this could be for the board recipient and how time intensive this could be for you (if you had to manually update each one!).
Rest assured, an edit on one project board will in no way edit that same item in a different project. These modifications are only within the same project.