Can't install cuda toolkit 6.5

I have a ASUS N551JQ laptop, which use Geforce 845M. I installed window 8.1, visual studio 2013 update 5.
I installed CUDA toolkit 7.5 and 6.0 without any problem, but i couldn’t setup CUDA toolkit 6.5, which use for my project (build opencv 2.4.9 with cuda 6.5 and vs 2013-i built vs2013+cuda 7.5+opencv 3.1 successfully but this version of opencv is not suitable for my project)
Any suggestion?

What was the problem with installing CUDA 6.5? Certainly it would be necessary to decline the driver install, as the driver bundled with the CUDA 6.5 toolkit for windows is not appropriate for your 845M. Therefore you should keep your existing driver, and deselect the option to install the driver.

The toolkit installer may give a warning that the installation is incomplete or something to that effect, or that it can’t find supported hardware (which is true), but it should still be usable.

i just found a solution there http://stackoverflow.com/questions/23037782/incompatibility-error-installing-cuda-on-windows
I will follow this guide and give you result later. Btw, tks for your comment