1. Uninstall from Control Panel.
2. Check if they offer Removal Tools for their products. (Known vendors like Avast, Norton and McAfee all offer some sort of tool).
3. Run CCleaner to clear safe Registry items.
4. For experienced users: Manual in-depth file/registry clean (can use Autoruns, Search Everything etc.)
I don't recommend using third-party uninstallers for security software, as often they require a reboot post-uninstall.
-close all running programs before uninstalling the antivirus
-block or disable the antivirus, if possible, before you uninstall it
-If you can't disable the antivirus, restart Windows in safe mode, then proceed to uninstall it as below
-to uninstall it, using preferably the same antivirus uninstaller
-If necessary when uninstalling restart your computer when prompted
-after you uninstall the antivirus to remove any traces left in antivirus computer using Ccleaner
-at the end of the uninstall process restart your computer.