This very much depends on which contacts you would like to delete whether it's a group or all of your contacts.
If it's a group of contacts that you already have in the system then all you will need to do is go in to the Group Section and select "Delete Group and ALL CONTACTS".
If you need to group your contacts then you will need to run a report or create a new group and select the relevant contacts before following the abovementioned steps.
And if it's all of your contacts depending on how you have entered your data, you can simply press the "Unload" button within the "Import Contacts" part of the system.