Many years ago I did change a tube, but I wish I had not started the job, and I was doing it along with a very knowledgeable B&O engineer.
The main issues are not so much the physical changing of the tube, but the subsequent setting up which, even for an experienced person is at best time consuming.
Also, as Peter says the voltages in the set when it is switch on are very high, about 30 K Volts on the tube anode as an example.
Frankly, unless you are very technically knowledgeable about modern TV systems, I would get the B&O dealer to do the work.