How to transition to mix feeding milk formula?

Hi mommies, 8 months na baby ko, sanay sya sa breastmilk direct latch and bottle feed, kaso humihina na yung breastmilk ko kaya nagttry kami iformula sya Bought s26, grabe pa expire na hindi pa rin nauubos. Ayaw nya talaga, tumitikim sya pero pag nalasahan nya, inaayawan nya talaga tapos hahanapin na dede ko. Helllp po thanks