Help with an undervolt.

[zCrazyeightz]

I bought the InWin A1 Plus ITX case a couple years ago, and I really like it. My problem is with the integrated 650W PSU. I'm using a 3090 XC3 that I bought off of a friend of mine. If I try to game for more than an hour or so, the whole system crashes. I'm sure that it's the PSU not being a high enough wattage for my GPU. My upgrade path is a little clumsy right now. If I were going to upgrade the PSU, I'd probably end up wanting to upgrade the CPU. If I'm gonna upgrade the CPU, I'll probably want a newer motherboard too. Which means newer case and so on. Until I decide I'm ready to pull the trigger on all of those things, I'm hoping for advice on how to undervolt my GPU to keep it stable. I've tried figuring it out on my own, but I just keep getting nowhere. Eventually, the system crashes to a black screen again.