Easiest way to save data on a file on github

Here's my situation: I've made a basic monthly budget webpage using html/css/javascript (that's all I know). I'd like to save one variable (an array of objects) to a file that's on github.

I got to the point where I managed to retrieve this data using fetch GET request, but I can't save this variable again to the file JSON file, updating it. Is that possible using only javascript?

I know nothing about PHP, nodeJS or other server-side languages or databases, I'm a newbie student.

What's the best solution for me? If the only options is to use a database and server-side language, what's the easiest, quickest way of accomplishing it? What's the quickest path for me to study and get this done in 48 hours?'

Thanks in advance!