For a carpenter it is a dead easy job, it might be simpler just to ask one to come in and do it.
For DIY, it depends how much you need to remove, what tools you have to hand, and how confident you feel using them.
In all cases take the doors off first.
If you need to remove 5mm or more then a sharp hardpoint handsaw will do the trick. A tenner or so from any DIY store. Mark the line and cut along it. You do need to be able to cut straight so a bit of practice may be in order first. You need to be quite confident to cut any less than 5mm with a handsaw.
For less than 5mm conventional wisdom says use a hand plane. However hand planes are not really easy things to use. Making them work properly is a bit of magic that most professionals learn very early on, and then assume everyone else understands too! So if you don't want to learn the black arts of plane fettling, sharpening and setting a somewhat alarming, but foolproof way, is to buy an electric planer for about Â£20-Â£30. Mark the line and make several passes with the planer until you have removed enough. Set the planer to cut 1mm (or less) on each pass. Quite noisy and very messy (an outside job) but unless you are confident with a saw or a hand plane it is easier. Again a bit of practice first won't go amiss.
One tip, keep your fingers where you can see them! Never put them underneath the planer when it is on â€“ it has happened, believe me.