Contacts can be updated individually, as long as you know the email address or mobile number. Navigate to Contacts > Manage and search for the contact in the search bar. If the contact exists it'll be displayed on the left-hand side of the screen. Check the checkbox next to the contact's name and click Edit in the Action Bar. You can edit the contact's properties in the Properties, More, and Custom fields tabs.
Contacts can also be updated using the Bulk Update feature. This function is particularly useful when updating the contact across more than one list, and you can update multiple contacts and their properties simultaneously. You can't update specific details, like email addresses and mobile phone numbers. Rather, it is used to update a more general property, like email status or a company's name. Bulk update is commonly used to change contact statuses, unsubscribe contacts, change subscriptions across lists, or to remove contacts from lists altogether.
You can update specific details for multiple contacts on a list by exporting that list to a CSV file, adding the new information into the file, and then re-importing the list into the system.