Oval, kidney, rectangle, olympic: you may not be able to change the shape of your pool, but if you want to change its look, the easiest and quickest way is to put in some new pool tile. Once you have all the materials on hand, this is a DIY project that you can tackle over a weekend.
Buy some new pool tile. You can either do this at your local home and garden store or online (see Resources below).
Turn off all electricity going to any items in the pool (light, filter, heater) before you drain the pool: the sudden loss of water would damage this equipment if it were still receiving an electrical current.
Drain your swimming pool using a submergible pump. You will want to check with your city first to find out if you will be able to drain your pool water into your street--as that water goes to the ocean--or if you must drain your pool water into the sewer.
Separate the old tile from the wall of the pool with a small flat chisel. Wear heavy-duty work gloves to protect your hands so you don't cut them with the chisel.
Chip away any harder to remove tiles by using a chipping hammer. Place the tip partially beneath any loose part of a stuck tile and carefully pry it free. Do this as gently as possible as you do not want to leave big gouges in the plaster where the tile was set.
Smooth over the surface with your trowels by using plaster or white thinset. Fill in any holes.
Apply white thinset to attach the tiles (see Resources below). Trowel in and smooth over the tiles--taking care not to move them--using corner and edge trowel tools.
Affix the grout using white thinset, once the tiles are set firm to the swimming pool.