Hi mohammad137603, glad to see you on Mozilla Support Forum.

You can update by following steps:-

• First, click on menu icon.
• then, click on options.
• In general menu, you will find firefox updates.
• click on check for updates.
• if update will be available then, you can easily download the update.

If it doesn't solved your problem, feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

If you have trouble with the upgrade, use this link; http://www.mozilla.org/en-US/firefox/all/