Humans are continuously breathing and consuming oxygen from the atmosphere. The used-up oxygen is replaced by new oxygen by the process of -

Photosynthesis is a process by which plants prepare their own food along with oxygen as a by-product. For photosynthesis, plants use carbon dioxide and water.

The carbon dioxide needed by plants is a product of respiration. Thus, when humans breathe, they take oxygen from the atmosphere and simultaneously release carbon dioxide into the atmosphere. 

This carbon dioxide is consumed by plants which then again release oxygen into the atmosphere. 

Thus, the used-up oxygen is replaced by new oxygen through the process of photosynthesis.