I'm unsure what you mean by user-agent, could you explain please?
The user agent that your browser is reporting to web sites.
Go to http://www.ipchicken.com
to see what your browser is reporting.
Use a user agent switcher in firefox or any other browser and change it to what you want.
Mozilla/5.0 (iPhone; CPU OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A5355d Safari/536.25
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:31.0) Gecko/20150401 Firefox/41.0
Mozilla/5.0 (X11; Linux; rv:2.0.1) Gecko/20140101 Firefox/41.0.1
Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0
Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2049.0 Safari/537.36
Next question is why would you want to do it.
To get pages or services made for a specific browser/ platform.
Such as mobile pages that are smaller and lighter. Or for a web site
that doesn't like certain browsers or platforms.