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

It will not make any difference.  If you do however, make sure it is "insurance proof" otherwise if something goes wrong and causes a fire, even though it may have nothing to do with the cable, they will use it as an excuse not to pay.  Not worth it....

You might look up Swagman. I was able to get rid of the wall wart on my Rega turntable with one of their power supply units.

The thing is, some turntables (e.g. many Project ones) require non-DC power supplies, in which case you're stuck with the supplied wallwart or a very close equivalent.

Why would you want to? Geeez it works make  no difference but hey it’s your money.