For users who do not have Zoho Mail Add-on: The email address used to send mails to leads or contacts in CRM is based on the primary email address of the user. You need to change your primary email address as per your requirements and the same will be used to send emails to leads, contacts, etc. See Also Changing Primary Email Address

For users who have Zoho Mail Add-on and who have configured the POP3/IMAP account: The email address used to send mails to leads or contacts in CRM is based on the email address selected from the Send Mail as drop-down list while configuring your mailbox in Zoho CRM.

Related Links:
How to change the primary email address in Zoho?