Simply login to the client area and download a full package of software, much like you would if doing a fresh install, and unzip the archive to your computer. It will create a folder named “ips_xxxxx” where xxxxx is a random string of characters. Then use an FTP client such as Filezilla and upload the contents of the ips_xxxxx folder (not the folder itself) to your forum main directory, overwriting when prompted and making sure all files transfer without error.
You will then go to your site and /admin/upgrade such as example.com/admin/upgrade and login with your AdminCP credentials. The upgrade system will scan you install to confirm that all files are up to date before proceeding. If it reports any are wrong then be sure you uploaded all files properly.
In 4.3 and higher, when logging into the upgrader, you will need to use an email address / display name and password using the standard, internal login handler. If you normally use a different method for logging into your community, you can bypass logging into the upgrader by adding this line to constants.php:
define( 'BYPASS_UPGRADER_LOGIN', TRUE );