The easiest way to turn Carbon dioxide into Oxygen is photsynthesis from plants

So, one way might be to keep plants warm enough in a greenhouse on Mars and slowly pump in the carbon dioxide you want to convert into oxygen. The plants would love it as long as they got enough water and sun and it didn't get too cold or windy where they were.

So, with enough plants in greenhouses on Mars (which would be a lot) you theoretically could begin to convert enough CO2 to Oxygen to support life and people living there.

However, there is no magnetosphere to speak of on Mars and unless people lived underground the lack of a magnetosphere likely would sterilize anyone on the ground if they weren't living underground deeper than 10 feet deep.

Because of the present Geomagnetic Reversal right here on earth above both polar circles the magnetosphere is often RIGHT NOW zero to 10% of what it was before 2000 AD. So, above the Polar circles life may be having the same problem if it is living above about 50 feet deep in the ocean.

This is likely true of life above both polar circles by the way North and South.