Update
You can use Opera Dragonfly, which is bundled with Opera any way.
You can invoke it by the Ctrl+Shift+I or by menu Page> Developer Tools> Opera dragonfly, then use the "Storage" tab (The "Cookies" sub tab should be the default), double click on an entry to edit it.
Source: Opera browser - cookie editor
For later versions of Opera, EditThisCookie is a cookie manager. You can add, delete, edit, search, protect and block cookies. EditThisCookie is also available for Chrome
From the plug in:
- EditThisCookie is a cookie manager. You can add, delete, edit, search,
protect and block cookies!
- Inspired by the lack of a good cookie manager in google chrome i
developed this small simple and extremely useful extension, that let's
you perform anything you might actually need to do with cookies
- ★ Delete any cookie ★ Edit any cookie ★ Add a new cookie ★ Search a
cookie ★ Protect a cookie (read-only) ★ Block cookies (cookie filter)
★ Export cookies in JSON, Netscape cookie file (perfect for wget and
curl), Perl::LPW ★ Import cookies in JSON ★ Limit the maximum
expiration date of any cookie