You can implement a simple function that traverses the object by passing a path, that you can take from splitting your object key on the dots.

Something like this:

setpath = ([p,...ps], v, o) => ps.length ? setpath(ps, v, o[p]) : o[p] = v


const data = { name: "John Max", plan: { active: true } }
const input = { fields: { "plan.active": false } }

let [path, val] = Object.entries(input.fields)[0]  // path="plan.active", val=false

setpath(path.split("."), val, data)
console.log(data)  // {name: "John Max", plan: {active: false}}

If you plan to have multiple path-like fields in under fields, and want to update all of them in the object, you can use Array.foreach:

Object.entries(input.fields).forEach(([p,v]) => setpath(p.split("."), v, data))

In case if you have multiple properties to modify in the object then you can create two reduce loops & modify the source data. Something like this:

const source =   {name: 'John Max', plan: {active: true}, newObj: {testObj: {name: 'Old name'}}};

const target = { fields: { "plan.active": false, "newObj.testObj.name": 'New Name' } };

const modify = (source, target) => {
    return Object.entries(target.fields).reduce((acc, [k, v]) => {
        k.split('.').reduce((a, e, i) => {
            a[e] = i == k.split('.').length - 1 ? v : {};
            return a[e];
        }, acc);
        return acc;
    }, source);
};
console.log(modify(source, target));

As you can see there are multiple nested objects & modify method will mutate & return the source object.

Note Unlike the setState method found in class components, useState does not automatically merge update objects. You can replicate this behavior by combining the function updater form with object spread syntax:

 setState(prevState => {
  // Object.assign would also work
  return {...prevState, ...updatedValues};
});