To customize an email for an individual prospect, please go to the Sequences page -> select the Preview tab -> select the intended prospect -> and customize any step you want to change. In the Preview tab, you can edit text, insert links, upload images, and attach files by clicking the corresponding icon in the toolbar. The changes won't affect the primary sequence template.
In the case of variables, you cannot alter them on this page because Reply populates variables with information taken directly from the contact, any attempted changes will revert.
If you want to customize A/B test emails, you can only change one email variation.