Firstly, make sure Auto Renew is enabled to have the option available for membership auto-renewal.

Members will have the option to set this up via a link in their Membership/User Profile and choose the option to Update Auto Renew.

1. Go to your membership/user profile and select the membership you want to auto-renew.

2. Click on Update Auto Renew

3. Read the instructions then click Turn On Auto Renew

Have another similar question? 

Learn How to Enable Memberships Auto Renew with Stripe?

How to Add a Form with Recurring Donations

Association Auto Renewal Membership Plans

Go here for the main help-files page.