How to delete old iPhone contacts?

My mum changes her number so I swapped it in my contacts book, deleted her old number and added her new one.

However when I go to text her old number still appears next to her name.

Anyone know how I can get rid of her old number completely?

Thanks

You probably saved two numbers for the same contact. Try to avoid that.

Well. If you have already deleted it in your contacts book. When you press edit and then delete. You need to sinc your iphone to your computer and any new messages that pop up on your computer you need to read and then one of them will say something like "things found on computer and not on iphone, sinc to iphone?" you need to click either cancel or NO and then wait for the sincing to finish.unplug your iphone and see if it worked. :) works for me