Step 1: Select the relevant email
Click on "Marketing" > "Templates" > The template to modify
Click on the pencil to modify the e-mail
Step 2: Insert a text block
Click on an existing test block or drag a new text block onto the e-mail template.
Step 3: Add the variable of the next payment in the mail
Click on the text block > "Merge variables" > "Subscription" > "Next billing date
Insert the variable in the desired location and click on "Save".
One last piece of advice, don't hesitate to do a test by sending yourself the email to ensure that the tag is correctly taken into account in your email.