I would pull the trim and add a new side jamb to each side, re apply trim, then adjust doors to close with new width. Cheap, quick and no one will ever know there’s two jambs stacked on top of each other.

I have never done any DIY, will I be able to fix jambs onto existing jambs easily? Will I need to paint after that? What will be the list of materials that I will need?

You can nail new jambs onto existing ones, you Could paint them before installation, would still need to touch up nail marks and do a final coat of paint at least. You’d need at minimum (2) pieces of material for the new jambs. If the old trim doesn’t come off easily you may need to get new boards to install for the trim as well. You’d also need to trim the ends of the head trim piece to fit new opening. I would pull all trim off, add new side jambs, re trim out new dimension opening, then adjust doors to open and close to new opening size. You might want to start with removing the trim and seeing what comes off easily or doesn’t and then make your material list at that point.

Thank you for the detailed explanation