If you still have half or a quarter of breast milk in the bottle after baby feeds, you can put it back in the fridge and offer this bottle again, but ONLY at your next feeding - after that, it should be disposed of and a fresh supply used.
"Used" breast milk can become contaminated when bacteria from your baby's mouth transfers into the bottle during feeding. If you find yourself with leftover milk after each feeding, use the volume indicator on the side of the bottle to measure how much they are regularly drinking and only put that much in each bottle, then add more once your baby begins regularly drinking it all.