Can you change the gpu on the lenovo h30?

If the motherboard has a spare PCI-E slot, than the answer is yes.

It has a PC-E x16 slot, but it's a low profile computer. Standard sized cards will not fit, you will be limited to low profile video cards. Also the PSU is a low wattage unit (around 250 watts) and it's a proprietary design. You can't use an off the shelf ATX PSU with this computer, which complicates things. You GPU options are very limited. A low profile Radeon 6570 or R7 240 are about the best you're going to be able to do. And neither one of those cards will run modern games very well.