Theoretically How can we make Mars' Atmosphere breathable?

To make Mars' atmosphere breathable, we would need to significantly increase its oxygen levels and reduce carbon dioxide concentrations. This could theoretically be achieved through terraforming methods, such as introducing photosynthetic organisms like algae or genetically engineered plants to produce oxygen. Another approach could involve releasing greenhouse gases to warm the planet, potentially allowing for the release of trapped CO2 from the soil, further thickening the atmosphere and enhancing its habitability. However, these processes would require vast resources and time, possibly spanning centuries to millennia.