Steps for customising the text displayed in WHMCS are as follows:

1. Open the /lang/english.php file in your preferred text editor.

2. Search for the text you wish to customise. Note down the corresponding string name, as highlighted below.

In this example we are going to change the text "Welcome to our Support Portal." So the string name is headertext.

3. Create the folder overrides within the lang directory.

2. Create or copy the language file you want to override. For example, to create an override for the English language you create ./lang/overrides/english.php

3. Open the newly created file in your preferred editor.

4. Start the file with a PHP tag, indicating PHP code is to be used:

5. Enter the string(s) you wish to override along with the new text.

In our example we would write the following into the /overrides/english.php file:

6. Save the file with UTF-8 encoding and upload to your server.

Further information is available at https://developers.whmcs.com/languages/