1. Login to your website
2. Click "CiviCRM" from the Administration Menu
3a. Go to "Contributions" --> "Manage Contribution Pages"
or
3b. Go to "Events" --> "Manage Events"
4. Find the contribution page or event you want to edit
5a. To the right of the contribution page, click on "Configure" --> Select "Contribution Amounts"
or
5b. To the right of the event page, click on "Configure" --> Select "Fees"
6. The corresponding page will have the list of payment processors available in CiviCRM
7. You can select one or more payment processor(s)
8. Once you've selected/enabled your payment processor(s), just scroll to the bottom and click "Save"
*Note: At this time Apple Pay is only applicable on Contribution pages, not events.