Not to put too fine a point on it, but:
If I know the When, I can handle the How.
Unless the How involves a slow debilitating syndrome which precludes my acting as I chose.
When for sure. First of all, like a previous poster pointed out, I don’t want to become afraid of anything, say, water-related or flying-related for the rest of my life just because I’ll eventually die that way.
But also, being the planner that I am, I think knowing my death date could help me plan out my life in such a way as to maximize my pleasure. You know, like if I know I’ll die at 40 I won’t plan for retirement, or if I know I’ll die at 80 then I’ll work a little harder at keeping my bones strong and maintaining my flexibility.
Knowing when is useful information that you can make plans around. Like, if I’m going to die at 55, I should really stop working my ass off to pay down the mortgage and save for retirement. Better to rack up debt and go see the world.
Knowing how is useless. The fact that Death has told you how means that you cannot avoid death by avoiding that situation/activity, but you don’t know when it is that you’ll die that way. For example, you might be told that you’ll die in a plane crash and avoid air travel your whole life, only to have a plane crash into your house.
Meanwhile, I think it would be human nature to be at least a little fearful of whatever will cause your Death, even if you knew the timing was pre-ordained. There’s no time for unhelpful fear.
What if you learn you’re going to die suffering horribly after a bout with some terrible disease. So what do you do if you don’t like the “how”? Per the OP, you can’t change it.
When.
Very useful for planning purposes.
There are some good arguments here for When, but I chose How. I could at least attempt to avoid the cause of my demise for as long as possible. Unless that’s just what Death wants me to think, that sick son of a bitch.
How could be interesting if the answer is something like Yellowstone mega-volcano eruption or mass extinction level meteor strike or nuclear bomb or grey goo incident, even if thankfully none of those events are very likely. Or it could give you nightmares for the rest of your life if the answer is something along the lines of burning to death. Or seriously cripple your life if it says you’ll die in a car crash and that makes you quit travelling by car.
What if your death is already certain?
If you learn you will die in a car accident and then avoid cars, can you actually avoid it or is it a matter of !Time’s Up!.
For the car accident, somebody comes up with a flying car and nails you in your top-of-the-mountain-with-no-roads-nyaa-nyaa! hovel.
What if the How is: ‘embolism in brain’.
Now what?
Are you going to never ever breathe again?
You’re going to die of smoke inhalation. Try to figure THAT one out.
Then I’d work on psychologically preparing myself for the suffering to come, like a prisoner on death row (who knows both the how and the when).
I really can’t see a downside to me knowing how. (Of course, YMMV). If I knew when, I guarantee I’d lose my mind from the anxiety of knowing each passing second is bringing me closer and closer to that day. But knowing how – even if it’s something horrible like dying from torture – would just motivate me to prepare myself for it with determination, and there would be a sense of relief knowing in advance that the torture will end and then I won’t be around to care about how much it hurts anymore.
I would rather know *where *than either when or how.
Then I just won’t go there.
Bad News: Each second IS bringing you closer to that day.
Given your user name, I’d think that you’d be more aware of that than most. 
Good Thinking!
You can out-wit Death!
“little dreamer, dream on…”
How. When could be tonight or 30 years from now and I wouldn’t care much one way or another. How on the other hand – it fate has something miserable planned for me I may just want to screw it over and take myself out.
Live ur life like its your last, knowing the how would be nice though.
Well, yeah. I mean, what could be easier than staying away from Stavromula Beta?
I misread the OP as “choose”, versus “know”, which yields a very different answer.
If I could choose “death by bungee jumping”, well, just avoid bridges + elastics and live forever.
That said, I guess I’d choose to know when, not how.
Yep. That’s what life insurance is for (we don’t have to disclose this information to the life insurance companies, right?)
Neither. I like surprises!
Robert Heinlein’s first published story, Life-Line, is about a guy who invents a machine that can tell you precisely when you will die, and how the insurance companies respond to this threat. (It’s not just his first published story; it’s the first story he ever wrote.)