Peugeot 508 1.6hdi 9HL Gearbox swap?! High RPM...

[przanowo]

Peugeot SW 508 1.6hdi 9HL 2011.
5 gear and having 2500rpm at 110km/h
What gearbox is compatible with my engine and with a longer fifth gear?
(just read that swapping a 6gear is to much work but with 5gear should work?)

[przanowo]

Is changing the final gearbox ratio or at least the fifth gear ratio possible?
New to this engine idk how the gearbox is assembled in this car.