Increasing the ratio of fissile Uranium (U-235) in natural uranium is known as :

Natural uranium contains only about 0.7 per cent of fissile U-235, the rest being mostly U-238. Raising the share of U-235, usually with gas centrifuges, is called enrichment; reactor fuel is typically enriched to about 3 to 5 per cent. Concentration is a different step: mined ore is processed into uranium concentrate (yellowcake), which does not change the isotope ratio.