It costed about 250 euros.

Is it easy to do by yourself or is it a job for the dealer

It is possible to upgrade the unit yourself provided you have the right very small slotted screwdriver to remove the three screws on the base and the two inside which holds the battery in place.

The biggest problem you will have is with the ribbon cable which is a push fit into the socket on the back of the key pad and is easierly pulled out from within the screen. To put the flat cable in the socket on the back of the keypad really needs plastic flat tongs to grip the cable without damaging it.

As you can also see below the flat ribbon cable is short which does not leave a lot of room when replacing the keypad.

If you do decide to do this upgrade yourself, make sure you are using a anti-static mat and strap

Before you decide to go ahead with a DIY job, check if the warranty is voided, and just how much you save by doing it yourself. It can't be that much you save, since you still have to buy the hardware.

Oh wow! I wish they can do this to their audio line too. 

Never seen an upgradeable remote in my life. And of all thing B&O could upgrade they chose the remote? 

This might be too late to help the original poster, but I thought I'd add some points from my experience going down this road:

1)  I had my Beo5 keypad repaired under warranty, and it was never quite the same.  There was a tiny bit a of play in the keypad, and it was no longer rock solid.  It wasn't bad enough to complain, but it was noticeable (by me, maybe not by somebody not familiar with the product).  This leads me to believe it is very difficult to get the Beo5 back together exactly as originally manufactured.

2)  The warranty on a repair is generally much less than on a new product.  In the US, I believe it is 90 days for a repair vs. 3 yrs for a new product.  I'm sure if you fix it yourself, this will void the warranty (if is currently under warranty).

3)  The difference between buying a new Beo6 and paying the repair center to fix my Beo5 (after my puppy chewed it up) was less than $100. 

From this, I decided against repairing my Beo5 and will just buy a new Beo6 when I can trust the dog to not chew it up.  Even the repair center agreed with this logic (which strengthened my opinion that the Beo5 must be a bear to fix).  DIY might change the economics a bit, but I'd be wary of trying to fix this one myself.

Stan