Bookmarks and Folders can be renamed by right-clicking on the name and then choosing "rename" from the context menu.

There is a search form provided where you entered your question which would have turned up the answer very quickly. ie. rename folder

You can change the name via the Properties in the right-click context menu or by selecting the parent folder (e.g. Bookmarks Menu) in the Library and select that bookmark folder in the right pane. You can then change the name in the Details pane at the bottom right.