You can change or add details saved in your contact entries. You can also set specific ringtones to specific contacts.
Some contact entries contain information from multiple sources: information that you added manually, information that Contacts joined from multiple accounts to consolidate duplicates, etc.
If you find duplicate entries in Contacts, you can join them into a single entry.
Changes made to information from one source do not automatically change the information on the other sources. For example, if you have information about a contact from a Google Account and an Exchange account and both are configured to sync contacts, changes to the contact from the Google Account are synced to that account on the web, but the information from the Exchange account will remain unchanged.