Renewal notices have to be setup for each membership level. Once
you have created the membership level, you can setup renewal notices to
be automatically sent to the members with that membership level before
and after their membership expires.

Notification type
You can select to send either an upcoming renewal notice (before the membership expires), or a past due notice (after the membership expires).
Send ... day(s) before/after
Specify here which day to send the notice to the member. Note that the notice will only be sent on that actual day (-0/+1 day). Notices will not be sent out retroactively, if the member's membership expiration date has already past the day, the notice will not be sent.
Applicable billing option
Here you can select whether the renewal notice is specific to a billing option, or if it would be sent to all members regardless of the billing option selected for the member. The benefit of having the renewal notices specific to a billing option is that you can provide payment instructions specific to that billing option for your members.
Email title and content
The title and content of the email you want sent to the members.
It is important to create your own renewal notices to maximize your renewal rates and revenue. Here are some tips:
To temporarily disable renewal notices you can set the number of days to 99999.
You can disable renewal notices to specific members by enabling the field "Do not send renewal notices" in their profiles. Note that you would first need to add this field to the Member Admin template under Customization. You can also add this field to the Member Manage template if you want members to enable that option themselves.