When sending an email directly to contact and not through any CRM automation, and you want to change the "to" and "from" in personal emails, follow these steps:

The "To" address in personal emails is the same as the email address associated with the contact profile. To change it, go to the contact's profile and update it with the new email address.

In a personal email, the "From" address is always the email address associated with the logged-in user. If contacts need to reach out in a personal email from a different address, this can be changed by changing the user's email address under Settings > Users. When the contact wants to respond, they will respond to the new email address.