can we change the cushion cover/upholstery in citroen c3? how? how much?

Yes. If you do not want to take a sewing machine and sew yourself some new ones, an automotive upholstery shop will be able to do this. Alternatively, you could get some seats with a different upholstery and install them. If they're not out of another C3, this may require drilling a few holes and a little bit of creativity, and you will want to measure the seat to be sure it fits.

If u mean a front seat base cushion/cover (the cushion & cover come as one) these r very east to replace, the only awkward bit is undoing the plastic trim as it is held in by a torx screw from the rear but it still is not a difficult job. A seat back Cushion is more difficult though as u have to remove the airbag with the cushion & fit it into the new cover. It still is not a really big job to do though. Not sure how much they are, l will try & find out tomorrow at work. l would check with a dealer anyway to find out the fitting cost, u may be pleasantly surprised!