My first piece of B&O and I still love it!
It is rather easy to open - two screws on the bottom and four to six on the rear panel, can't remember - but there are a couple of things to watch for. You will first need to disconnect the built-in antenna from the rear panel connector (if you still have it, it's often cut off or at least missing the connector). Then push carefully from the front panel and the chassis will slide out through the back, but on a 900K, you will need to disconnect the built in speakers halfway out. It's a single connector and should be pretty obvious.
It's possible that both dial light bulbs are actually out, but it's very common that they don't work because the fuse has corroded in its holder (the dial lamps have their own fuse, which doesn't power anything else). Clean & replace if needed, but be prepared that the metal connectors of the fuse holder might crack. Then you need to replace it as well.