You cannot re-block this specific person for 48 hours after unblocking them.
Find the person you want to unblock under Tap the blue “Unblock” button next to their name.
To unblock a profile on the Facebook mobile app, you must navigate to the section within your account settings. This process is generally the same for both Android and iOS devices, though the initial menu button location may vary. Steps to Unblock on Mobile How To Unblock People On Facebook
The unblocking feature on mobile is a classic example of Facebook’s "if it ain't broke, hide it deeper in the menu" design philosophy. It works perfectly 100% of the time, but it fails the usability test because it forces users to navigate through three layers of settings to perform a simple social action.