I plugged my iphone 5 into someone else's computer and all my contacts and email were replaced by the other persons information. How do I reverse the sync?
Are you saying that you did NOT get a prompt that indicated that your device was new to the iTunes app on that computer and that it just transferred the computer owner's data onto your device without any prompting from you?
There are a few questions you need to tell us first. Do you sync your own contacts with a computer? Do you use iCloud? When you connect to iTunes under the second tab at the bottom should be an option to replace what is on the phone with what is on the computer(I'm not currently connected so I'm a little fuzzy on it). Otherwise connect to iTunes and do a sync on your own computer.