How to upgrade "power cord" on turntable with wall wart?

If it’s a wall wart you can’t change the mains cable on it as it hadn’t got one. You could try an alternative if you know what it is supplying the table, easy enough if it’s AC and you know the voltage and frequency. As a first test that won’t cost a penny try the supply you have in different positions on the power strip, assuming you’re using one. If you can hear position make a difference than playing with the supply might too. A larger transformer maybe if that’s all the wall wart is. Naim used to make a supply for the Linn LP12 that was just a large toroid in a box and a phase board that went in the deck. I used to power a Manticore Mantra from an isoformer instead directly from the mains to good effect but when I bought a Townshend Rock with a Merlin ps (producing a synthetic sine wave) the isoformer degraded the sound when I put it before the ps.