Actually, at least in some jurisdictions, you do. What you don't have is an obligation that device vendor helps you - unless they took some obligations to do so (and GPL is one).
But it's entirely another story that they had chosen to use GNU or Linux bits in their software.
I absolutely have every right in the world to do whatever I want with any device I own (baring things like tweaking WiFi to violate FCC rules). Why would you even say that?
Yes. In fact, isn't this exactly why GPLv3 has the anti-TiVoization clause? Because TiVo under GPLv2 were entirely in their right to deny user-modified GPL-licensed software from running on their hardware.
Under GPLv3 you have the right to modify the device you purchased if the manufacturer (or the person you bought it from) has the same ability. Linus doesn't like the GPLv3 for what appears to be I-dont-care-about-user-freedom reasons, which is why only parts of the kernel are under GPLv2-or-later. But that's the main reason why I make all of my projects GPLv3, so that users are protected in this instance as well.
How? You don't have a right to modify the device you purchased. You don't have a right to manufacture a clone of the device and run your patched code on it. The vendor can happily decline patches from you. So can Linus.
All you will have is the right to inspect code they used and learn from it or reuse it in a lightbulb you choose to manufacture that doesn't violate some other IP the original vendor has on the original bulb.
There is absolutely nothing about the GPL that grants you any particular freedom regarding the specific light bulb you purchased. If it sucks, it will continue to suck.