What is Psu? can i find it with the model?
PSU is the Power Supply Unit. It's what delivers power to the system. You can read about 'Computer Power Supplies' if you do a Google search, and yes, you can find your one using the model number.
After looking yours up, it appears to be a unknown brand 500W PSU. Its worth mentioning that a good quality PSU is needed for your components to run properly. if its of bad quality, the PSU can end up damaging other components or leading to system crashes!
I wont ask you to check the branding of it, as most prebuilts don't add good quality PSUs. They also go with lower wattage PSUs, so your upgrades are limited.
I do not recommend an unknown 500W PSU at all on a 40 series card (or any expensive card/system, actually), so you will need to upgrade the PSU as well if you need a GPU upgrade. Adding to this, most Lenovo motherboards (i couldn't find yours specifically though) use proprietary power connectors for the motherboard, which means you're stuck with the PSU they gave you (unless they have a higher wattage one for sale), if yours is proprietary.