There's no way to set it as the default, but if you just have to see the Most Recent posts first, you have an option.
It seems as though Facebook's interface is ever-changing, but the latest UI overhaul hitting devices is pretty dramatic. It's mostly for the best, bringing a cleaner look and surfacing more features rather than hiding them behind obscure menus. A few things have become harder to find, however — one of which is the "Most Recent" News Feed view.
The newly-redesigned Facebook app now only shows the "Top Stories" view for your News Feed, which surfaces content that it thinks is most relevant to you, rather than what's been posted or interacted with most recently. In the old app you could simply change the setting from the default to show Most Recent in the main view, but now you'll have to do a little digging.
To view the Most Recent feed, tap or swipe over to the far right tab (three horizontal lines, titled "More") of the Facebook app, and scroll down to the "Feeds" section, right under "Apps." Here, you'll see any different feeds you've created with friend or family groups, but you'll also see "Most Recent" as an option. Tap that entry and you'll be shown a completely interactive view of your News Feed based on time rather than some other algorithm. You can go back to the settings page by hitting the back button or tapping the "News Feed" button in the top left corner of the app.
Now it should be noted that no matter what you do the main News Feed view of the new app will always be Top Stories, so after you close out of the Most Recent view in the settings you'll be back to the old sorting method until you manually go view it again. It's not the best situation ever, but at least you have that option if you want to spend the few extra taps to do it.