installing gem from a github fork

I have installed a mongify gem from original source: https://github.com/anlek/mongify

then I want to install a newer version from a fork (has not been accepted yet for pull by original source, but it has an update that i need),

link to the fork: https://github.com/yuvalkarmi/mongify

what should I do to update mongify in my device from the fork (not from the original source)?

Thanks