The Basic steps to install 64 bit Windows 10 are as follows. I assume here that your current configuration is the default “Out of the Box” OS X installation.
- Download the correct Boot Camp Support Software for your Mac. Transfer these files to a FAT formatted flash drive. For your Mac, use the same software as for a 64 bit Windows 7 installation. These files can be found at Boot Camp Support Software 5.1.5621
- Use the Disk Utility to reduce the size of your OS X partition. In the empty space, create a FAT formatted partition with the label BOOTCAMP.
- Burn the Windows 10 iso file to a DVD. You can use the Disk Utility to do this. I always use a RW DVD. This takes longer to burn, but is reusable.
- Restart the Mac with the DVD in the optical drive. Hold down the alt/option key and select the DVD icon with the label Windows.
- Install Windows to the Boot Camp partition. You will have to tell the installer to reformat this partition as NTFS. DO NOT CHANGE THE PARTITIONING DURING THE INSTALLATION PROCESS.
- At some point during the installation, the computer will reboot. When it does, it will boot to OS X. This is to be expected. Go to System Preferences and instruct OS X to boot back to Windows. The installation process will continue.
- After the Windows installation has completed, install the Boot Camp Support software stored on the flash drive. To accomplish this, you will have to right click on the icon and select Compatibility Mode.
I do not consider the process to be that difficult. The most challenging step is creating the FAT formatted partition labeled BOOTCAMP. If something goes wrong, you are in danger of loosing data on your OS X partition.
Source : StackExchange