Be sure to check out our article on How to Generate Your Tracking Links for a more in-depth explanation.
Not all offers will have alternate landing pages, but for the ones that do have them, all you need to do is click the URL dropdown when generating your tracking link:
The dropdown will expand to show all available landing pages. The default link is referred to as the "Base Destination URL" - all of the other options are alternate landing pages:
When you select an alternate landing page, your tracking link will be updated automatically - clicking it will bring you to the chosen LP.
If you do not see any options besides the "Base Destination URL", it means that offer does not have alternate landing pages available.