Go to the support page here and download the chipset drivers and the Intel Rapid Storage Technology driver.
I understand Dell does not support Windows 10 on the Precision M6400, but I love this machine and a number of people posted that they tried it and it seems to work fine.  I upgraded mine and I agree, initially it seems to be working fine; however, there are a few items in Device Manager that look to be not happy.  There are 3 items called "Base System Device" and one item called "Broadcom USH" that are showing issues. PCI\VEN_1180&DEV_0852&SUBSYS_02511028&REV_11\4&31182d7c&0&0DF0 PCI\VEN_1180&DEV_0843&SUBSYS_02511028&REV_11\4&31182d7c&0&0BF0 PCI\VEN_1180&DEV_0592&SUBSYS_02511028&REV_11\4&31182d7c&0&0CF0 USB\VID_0A5C&PID_5800&MI_00\6&66de6c9&0&0000 Any help finding drivers for these devices that would be compatible with Windows 10 64-bit would be greatly appreciated.
Dell Precision M6400 Running Windows 10 - Missing Drivers