How to remove Bosch 650 chuck

I forced my drill too much when using in a drill guide so something strange happened to the chuck. At first it didn't turn easily to lock and unlock there were click sounds when I was trying to turn it with the help of lock button at the top, then now it completely stopped turning so I have to replace it.

I tried the traditional method mentioned on manual ( using an Allen key) but not working. I took it to the service and the man there couldn't do that with the same method. And he said I have to open this up to remove the chuck and he wanted to charge me a little more to do it. So I didn't want it. I decided to remove it myself.

I opened that before to remove the chuck but I couldn't find a way. Is it easy to remove it after opening up the casing? If so how? Could anybody help? Thanks in advance.