How to open Oasis 3 to change battery?

How to open Oasis 3 to change battery?

I would like to know how to open the device to change the battery. At the moment it is not necessary but because the battery runtime is about the same on my 6 month old Oasis 3 as on my 6 year old (heavily used) Paperwhite i bet that i will have to change the battery after ~3 years.