I've fairly new in computer my problem is that I have a Lenovo m900 sff and I wanted to upgrade its TFX PSU to an ATX one since atx has a higher voltage so I can add a dedicated GPU. will this cause any issue for my Lenovo motherboard?
If you have a SFF Lenovo system with a TFX power supply, you cannot PUT an ATX power supply into that case. It will not fit. And there is a very good chance that your supposedly TFX power supply MIGHT be proprietary as well. Even if it isn't, you can't put an ATX PSU into a SFF case.
it's very likely that this motherboard and power supply are sharing OEM proprietary connections.
this would mean that even if you could somehow fit an ATX PSU into the mix,
it still wouldn't be compatible with this motherboard.
your best option for "upgrading" this system is likely to just salvage the few components that may be worth keeping and building a new system in a proper ATX case with retail power supply and motherboard.
I don't think he has the tower type as he specifically says he has the SFF M900 which means it only has a 210w power supply and it also means that an ATX power supply can't be used. So even without the potential issue of proprietary connections, they are still going to be limited to a TFX form factor power supply. This is why buying SFF OEM style prebuilt systems is a bad idea if you have any thoughts of upgrading it down the road.
There are a few options out there if somebody is determined, like the Silverstone TX500-G and TX700-G, but they are not cheap at like 150 and 190 dollars respectively. TFX isn't a popular form factor except for in slim OEM prebuilt systems so there aren't a LOT of options out there over 300w.