JSFiddle - React, Tailwind, and code Playground

JavaScript

var ob = {
    "first": "first-string",
    "second-array": [
        {
           "first-Obj": 2
        }
     ]
};

function changeTheValueAt(obj, path, value) {
    var parts = path.split("/"),
        final = obj[parts[0]],
        i;
    
    for (i = 1; i < parts.length; i++) {
        if (i + 1 < parts.length) {
            final = final[parts[i]];
        }
        else {
            final[parts[i]] = value;
        }
    }
} 

changeTheValueAt(ob, "second-array/0/first-Obj", 1000)
alert(ob["second-array"][0]["first-Obj"]);