A Superseded Pay History Line shows a pay change that was requested which was not approved. Because you can't have the same effective date on a Pay History Line, only one line can be the active one.
As the approved one is active, the Superseded line is present only as a formality and will not adversely affect an Employee Record. You don't have to remove it if you do not wish to.
If you do, you will not be able to open the superseded line. But if you click on the line below to open and delete this, it will then remove both the approved and superseded lines.
📌 Note: When deleting the line you may get an error message. You can close this error message and press F5 on your keyboard, which will refresh your screen and the lines will be deleted.
If you then click Add Payment (Admin - Unrestricted) on the bottom right, you can add a new line for the correct date and information which will show as a single line.