The best way to go about this is to tell her in person. Using any other means is disrespectful and embarrassing. Do not do this over the phone, through a text or through an email.
Have a conversation with her, let her know you no longer feel the same as you use to feel, let her know you respect her but something in you has changed. Let her know how sorry you are to hurt her but you have to be honest and move on from the relationship.
The best way to do this is to do to face-to-face. You cannot tell someone that you do not like her over text. That is a disgrace. The best way to do the deed is to sit down with her and tell her why you are not able to continue the relation. You do not have to be direct and say that you do not like her.
Try telling her how she is much better than what you deserve. Try to compensate by letting her vent her feelings. However, you need to be clear that you are breaking us so that she knows that this is the end of the relation.