On every membership transaction, there is an Exp Date field that can be used to determine when that specific transaction will expire. You will also find a column on the Memberships grid called # of days until expiration. This column can be helpful if you don’t wish to filter by a specific date range but rather everyone who expires within a certain number of days. You can filter the list to identify the groups of constituents you would like to send a renewal reminder to and use the Actions menu to send a mail merge letter, text, or direct email.
While looking at the Exp Date, it might be worth also paying attention to the Current Expiration Date column. If you see a different date listed there, this means this constituent has a newer membership transaction with a different expiration date that is farther into the future.