The Unearthed Arcana Scout archetype for Fighters gains Natural Explorer at 3rd level. It is written as the PHB Ranger feature. Can a Scout now use the UA Revised Ranger's Natural Explorer feature instead?
Unearthed Arcarna are all unofficial rules and only usable by the agreement of the players at each particular table, particularly the DM.
You can think of the material presented in this series as similar to the first wave of the fifth edition playtest. These game mechanics are in draft form, usable in your campaign but not fully tempered by playtests and design iterations. They are highly volatile and might be unstable; if you use them, be ready to rule on any issues that come up. They’re written in pencil, not ink.
In the case where an unofficial rule1 cites an official rule2 which is then superseded by another unofficial rule3 then if you are going to use the first unofficial rule1 then you need to consider as a table what the implications are of changing to the second unofficial rule3 and make your decision.
1 - Unearthed Arcana "Kits of Old", Fighter Archetype: Scout incorporates the Ranger class feature Natural Explorer.
2 - Ranger class feature as published in the PHB.
3 - Ranger class feature as published in Unearthed Arcana "Revised Ranger".
Putting aside the fact that both the Scout archetype from UA Kits of Old and the UA Revised Ranger are not "legal" classes in the normal sense, you cannot swap one class' features with another class' features, even if they have the same names, unless your DM has allowed it.
The Scout archetype was clearly written using the PHB Ranger's Natural Explorer feature in mind, given that it was written before the Ranger, Revised article:
At 3rd level, you gain the ranger class feature of the same name, with the following alteration: You choose additional favored terrain types at 7th and 15th level.
In addition, the UA Revised Ranger's Natural Explorer feature does not include a Favored Terrain option at all, and based on the description of the feature on the Scout archetype it seems clear the Scout is focused on the survival and terrain exploration features of the PHB ranger, not the martial aspects of that class.
If your DM (or you, if you're the DM) has decided that some or all of the Unearthed Arcana character options are allowed, you still cannot swap the Scout archetype's Natural Explorer feature with the Revised Ranger's Natural Explorer feature, because class features for each class are unique to that class and only that class, and the Scout was written with balance against the PHB's ranger in mind.
The UA Revised Ranger is a new class. It does not update the Ranger in the PHB, nor does it update any other class. The features defined in the UA Ranger article only apply to the UA Ranger. You could play either the original PHB Ranger or the new UA Ranger, but not swap features between them--at that point you're making a new, custom class.
The same could be said about using the UA Ranger's Natural Explorer feature with the UA Scout archetype; you're no longer using the Scout as written (because it was intended to use the PHB ranger's Natural Explorer Feature) but instead are making some custom homebrew class, which could upset the balance of the Scout archetype in comparison to the PHB Ranger or even other PHB Fighter archetypes.
That being said, as Dale has pointed out in his answer, you're already well beyond official rules territory here, so it really is up to you and your DM and the rest of your group how you intend to proceed.