What should I upgrade?

[Indoanal]

I have a lenovo ideacentre 700-25ish, the specs are
Intel Core i5 6400 (2.7 GHz) 8 GB DDR4 1 TB HDD 120 GB SSD NVIDIA GeForce GTX 750 Ti 2 GB Windows 10 Home 64-Bit.
I have a lot of problems playing most higher end games and I don't know what graphics card I should get or if I should even upgrade the graphics card.

 

[Barty1884]

Can you describe the "problems"? A 750TI isn't going to be maxing out modern titles or anything, but should certainly still be able to run them without issue on low(er) settings.

If you do need a GPU upgrade, a 1050TI is a decent step up..... without increasing the power draw (a concern in prebuilts).

Without knowing the spec of the PSU in that system, it's impossible to know if anything more substantial would give you problems or not.

 

[jaybatick]

You could try updating your drivers.