How do I replace the ink cartridge slider

Try lifting up the top, but by pulling up along the sides of where the glass is. It should usually just roll right up. If it doesn't Don't force it. Stop there

Otherwise there's likely a panel that moves out of the way just above the paper tray.

Once you've got it open, most printers these days will move the printer heads (with ink cartridges perched on top) into an accessible position. Then just rock the head out and do the reverse motion to get the new one in. Just double check you've removed all plastics from the new cartridge or you'll have to do the whole thing over again when it doesn't print right, or at all.