If you can kick off self-sustaining biological processes it’ll happen on its own eventually, but you’d just be looking at generational time scales to do it.
Of course you’ll probably have lots of side-effects.
How do we do that? I imagine dumping Earth life on Mars it will just die. What if we buried a terrarium at the Martian pole with a radio isotope and solar heater and controls so that it could try growing bacteria inside and controlled-leaking some outside into a nearby warm (liquid water) surroundings, and that could get many chances to evolve strains that could survive further away - analogous to ocean life around deep hydrothermal vents.
The closest thing I’ve heard of is genetically engineering some extremophile bacteria to seed some bacterial colonies with, and then building some domed habitats and dropping plants and fungi into it. And then you could allow some of that life to grow outside the dome and spread over time, slowly bulking up the oxygen levels. There are also proposals that you could have orbital focusing mirrors to beam additional heat at the planet and start to melt some of the water and frozen greenhouse gases.
Any effort is only temporary though, because Mars doesn’t have a magnetic field, so the sun is constantly stripping atmosphere off it. It’s not a renewable resource there like it is here.
Anyone know of speculative plans of this sort?