Setting a value on a rollup field is impossible in JavaScript in CRM 2015. You can’t refresh the field individually without clicking the Refresh button — which can be a pain for users if you have several rollup fields to recalculate.

function setValueOnRollup(field, value) {
//unsupported way of putting a value in the calculated field
//DOES NOT update the value in CRM, only on the display
$('#' + field).find('.rollup').find('span')[0].textContent = value;
}

So if you wanted to use it on a form, put setValueOnRollup(‘yourfieldhere’, ‘100.00’)

It’s best used when you have the values you want to show (preferably the real values) which can be retrieved with a query you can make with CRM REST Builder (https://crmrestbuilder.codeplex.com/).

These also might come in handy to manage the rollups and processes:

Process.js – CRM 2013/2015 Call Action, Workflow, or Dialog from JavaScript (https://processjs.codeplex.com/) is a cool tool to run a process from your custom JavaScript.

Dynamics CRM 2016 Workflow Tools (https://msdyncrmworkflowtools.codeplex.com/), specifically the “Force Calculate Rollup Field” feature to update the value in CRM.