Go and buy some nice new compost - multi purpose is fine. Make sure you have pots just a little bigger than the ones they are in at the moment - not hugely bigger. Gently remove plant from pot by holding pot upside down, remembering to support plant with your hand and shake it free. Remove as much old compost from around the roots as you can - gently. Fill new pot about half full of new compost. place plant in pot to check levels. It shouldn't stand proud of the rim of the pot nor should it be too far down. When you are satisfied that the level is right fill the pot with compost, firming it in as you go - this means pressing down with your fingers around the sides so that you leave no air pockets. Do it firmly but no need to ram it in. Leave at least 2cms clear at the top of the pot otherwise when you water compost will cascade over the sides making a mess. Water thoroughly. Easy peasy.