Changing the PSU involves a lot of bolts and nuts! You need to take the casework apart, remove the PSU cage assembly, and extract the PSU from the cage.
If you look at the internal pictures on Duncan's site, you'll see what I mean.

There are two connectors - one for the mains power and one for the output voltages.

There should not be any need to solder anything.

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.