If I in Turtel Games (Fortnite) more FPS would rather buy new graphics card or processor?

Graphics card: Nvidia Geforce Gtx 750 2GB
Processor: Intel Core i5 6400

Since you did not name your hardware, nobody can tell you that.

Depends on what you have just installed. But I would rather take the older of the two

In most cases, a new processor, but since your graphics card is very old you will probably have to renew both. Since you're always good at low graphics when you're good and at lower graphics you do not need a good graphics card, as a good CPU is much more important!

Both outdated, it is best to renew both, but if that is too expensive for you, I would resort to a procesoor with indigrated grafim unit in your place