Are the walls insulated? If not, do that first. Most of the heat escapes there...especially the part nearest and above the ground level.
With a fully insulated wall, you might not even need to insulate the slab. in MN we insulated our walls and then left the concrete floor as-is (stained it). Only on the coldest winter days did we need to turn on the fireplace and when we did, the slab was fine comfort-wise.
Otherwise, the most typical way to insulate the slab will be with foam board. Typically EPS or XPS with a floated plywood subfloor on top.
But if you have no intention of ever finishing the basement, maybe insulating the first floor underside might make more sense.
The above is valid for a house with a basement. I misread your question initially. It appears you have a floating concrete slab as your foundation. To insulate that you need insulation...either under it (preferred) or on top of it. Obviously adding insulation underneath is not practical after the fact.
The cheapest solution is likely just a new floor covering...carpet with a think pad...maybe cork, etc.
More complicated, but doable, would be to lay down EPS or XPS and then a floor on top of that. The trick there is that this will change the height of your floor...likely causing issues with doors, thresholds, cabinets, etc.