What are the rules updates for Catan in the 5th edition? Are there any?
Changes to the base game

The biggest change in the base game is that the name has switched from "Settlers of Catan" to just "Catan".  They've also updated the art (citation).

Changes to seafarers

Many of the scenario maps are different.  It seems they used the maps from the 2008 German printing of seafarers when making 5th edition.  The big change is that there are now separate maps for 3 players vs 4 players on the first four scenarios.  Additionally, the layouts of "The Fog Islands" has been completely changed.  For a look at historical Catan scenarios, see this compilation.



Prior to fifth edition

Between third and fourth edition of base Catan, they added a rule allowing you to move the robber to the desert, and renamed the "Soldier" card to a "Knight", presumably for terminology consistency with Cities and Knights.

Between third and fourth edition of Seafarers, they changed the game border from rectangular to hexagonal, meaning all of the scenario maps got changed then as well.

Also, see What are the differences between the four editions of The Settlers of Catan base game?