Not having a crystal ball it is difficult to know if you mean in webmail a mail client or mail app.
If you mean webmail, just click on 'Contacts' at the top of the page, hover your mouse over the contact you wish to delete and then click on the 'dustbin' delete icon on the right. If you want to delete multiple contacts, tick the box against the contacts you want to delete, click on the 'More' dropdown and select Delete.
