Thank you for reaching out to the Community.
When you say you want to personalize an e-mail do you mean including a greeting message (i.e. "Dear John") unique to each e-mail address? This FAQ will walk you through inserting the unique greeting.
If you want to send a reminder you can either create a new campaign or make a copy of the original e-mail you sent regarding whatever it is you want to remind them of. Then schedule the e-mail to the contacts you sent the original promotion to.
If you're referring to a discount you're offering in your e-mail then all you would need to do is update the language in the e-mail to reflect the extension of the discount.