how do i update my mac to where i can get the darkmode

If your mac is up to it, see Section 1,

How to upgrade to macOS Mojave - Apple Support

then if you want to go for it see Section 4.

Troy- wrote:

how do i update my mac to where i can get the darkmode

You do not say what exact Mac /model /year /size /retina /touchbar is this

or

what exact macOS you are currently running?