1) Download the latest version of WHMCS from here: https://download.whmcs.com
2) Extract the zip
3) There you will find "WHMCS" folder, compress it
4) Upload it to the WHMCS directory where it is running i.e https://yourdomain.com/whmcs orhttps://whmcs.yourdomain.com, whichever is running under the folder of your WHMCS.
5) Extract the "WHMCS" folder which is uploaded to replace all the existing files to new files.
6) Visit your WHMCS URL i.e https://yourdomain.com/whmcs/install - replacing your-yourdomain.com with your actual domain.
7) Find the file name "configuration.php.new" and rename it to "configuration.php"
8) Remove the "install" folder.
9) Click the I Agree button to accept the license agreement.
10) WHMCS will perform some System Requirements checks and if all pre-requisite steps have been completed successfully it should pass.
11) Click Begin Installation to continue.
12) You will now need your WHMCS license key, and the details of the database and database user you created in the pre-requisites.
13) You will now be prompted to setup the Admin user.
14) Once the installation completes you'll see the final screen detailing some further actions that need to be completed before you can start using WHMCS.