The Object Manager is displayed on the left side of the plot window, highlighted in this image.
If the Object Manager is not visible, check the box next to the View | Display | Object Manager command.
Right-click on the Line/Scatter Plot, or plot you would like to rename, in the Object Manager and select Rename Object. Type a new ID into the box and click OK. Alternatively, slow double-click on the Line/Scatter Plot object name in the Object Manager to edit the ID.
Updated March 9, 2020