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.