Have to give a talk soon but I no longer believe in my research Basically, my supervisor wanted to measure variable X. X is very important to him due to some other projects we are working on. So he wanted me to give a talk on measuring X at an upcoming conference.

I was skeptical -- I thought that maybe X wasn't a real variable, but just a mathematical effect that would cancel out if we did our measurements right. My supervisor gave me some compelling arguments and convinced me that X was real and we should measure it, so I submitted an abstract to the conference and got accepted.

Now, having done the work and taken the data, I am once more convinced of my old idea that X isn't a real physical thing. The talk is coming up soon and despite the enthusiasm that appears in my abstract the best I can honestly say is that we took some measurements and got no results. I know my supervisor would be upset if I give a talk to the effect of "X doesn't matter" because that reflects negatively on his other projects. Should I cancel the talk, change the content so it's drastically different than the abstract, or something else?
Do a practice talk with your advisor before going to the conference. Do it early. It will be a good time to discuss how the talk should be organized in order to  highlight the context of your measurement in the whole project and its relative importance. 

It is common for a student working on a small piece of problem (and not necessarily looking at its entire context), to go back and forth between feeling A) the piece is obvious or unimportant; to B) the piece is pretty cool after all; then back to A again. 

At worst - if it seems as if your piece really is not so relevant, then I expect he will help you organize the talk so that it still engages the audience. (Not a bad skill to practice. And, if you don't pull it off - at least in my field, if the topic of the talk seems unimportant to the audience, the bad smell lingers longer on the professor/advisor than on the student).

(Of course, also in my subfield, giving a talk at a conference or workshop usually means that we are giving a talk about work-in-progress still unpublished. So strategies may be different if there is a required paper involved for every talk).

But more likely, with working on the practice talk and the discussion with the advisor - you will develop more of an appreciation of how that small piece of work fits and may even develop an appreciation of its other subtleties that interest the professor. In a situation as you describe, I place my bets on the professor instinct on the topic.