You should backup the already existing KSP.exe before that. There you copy the 'player_win.exe' to the KSP folder, and rename it 'KSP.exe'. After you installed it, go to the installation folder and to the path 'Editor/Data/PlaybackEngines/windowsstandalonesupport/Variations/win64_nondevelopment' Download it from and install it, since I can't redistribute the 2 files you'll need alone.

You'll need Unity version 4.6.4, which is the version KSP 1.0 uses. Try this on a separate install of KSP, or back up the game files before doing anything. Please don't confuse Squad or modders with bugs that have nothing to do with what they are giving you. Squad had good reasons not to release a 64 bit version with 1.0, and I'd like to keep the win64 bit discussion in this thread only.

Also, PLEASE do not report any bugs you find in this workaround as bugs with the game. There is no real advantage of 64 bit, other than the increased memory capabilities, so think twice if you need it. You should really only try it if you use a bajillion mods, and run out of memory otherwise.

Also, should you try this I do not take any responsibility on any of the results of you doing so. I haven't had many problems with previous (official or non-official) 64 bit versions of KSP, so this still might crash on start up for you. Now, I haven't tested this much, and some bugs and crashes certainly are still very common with this (for instance the left mouse button works as right mouse button when leaving the game thingy), but the game launches in 64 bit just fine for me. So, since 1.0 has no official Win64 version, because of obvious instability and buggyness of Unity in 64 bit, I though I might as well try whether the old community hack we used back in.

Ksp 64 bit mods