For iOS devices
Tap and hold on a one-to-one chat > Tap Select Chats > Tap to select any other one-to-one chats > Tap Delete > Delete Chat

For Android devices
Tap and hold on a one-to-one chat > Tap to select any other one-to-one chat > Tap > Delete (tick the checkbox if you also want to unfriend the selected contact)

Please note that all messages and media files inside those chats will be deleted and cannot be reverted.