GPU_FORCE_64BIT_PTR 0
So write "set" before the GPU....stuff and you're good to go. I like to use "setx" with those commands in a separate bat and not set them everytime I start mining.
I never use setx because it affects every program running later, not just miner. And you may not need them same for all miners. But for miner it is enough to set them in the current shell that runs this bat file.
The syntax of set and setx is different: for setx you do NOT need '=', but for set you do (thanks Microsoft for 'consistency'):
setx GPU_FORCE_64BIT_PTR 0
or
set GPU_FORCE_64BIT_PTR=0
Again, I will never spoil the global environment with setx unless I really need that. And in this particular case you hardly need it since anyway you run the miner from the bat file. So simple set is preferred.
And yes, set /? or setx /? always help

Interesting information, I didn't know that, thanks for sharing! I'm curious, what are the unmodified values for GPU_FORCE_64BIT_PTR, GPU_MAX_HEAP_SIZE, GPU_USE_SYNC_OBJECTS, GPU_MAX_ALLOC_PERCENT, GPU_SINGLE_ALLOC_PERCENT or is there a way to reset them to values before setx command? I ask this because I'm currently mining with my daily pc that i play games etc. Would any of these commands affect gaming etc. negatively?