You can either download the script via the releases section, clone the repository, copy-paste the code like a degenerate milksop, or — if you're on Mac — install via Homebrew:
$ brew tap solidiquis/tap
$ brew install alacritty_font_size
I would most definitely recommend creating an alias for usage in the command line as the name is quite long.
<Up> | kto increment font-size.<Down> | jto decrement font-size.
Exit with Ctrl-c.