The default text is at the top of a SharePoint Site is “SharePoint”. You can change it on the web application level by setting a property value for SuiteNavBrandingText.
View the current header text
$web.Get-SPWebApplication “https://CentralAdminURL” | $web.SuiteNavBrandingText
Note – PowerShell Returns a blank outer space looking response…don’t be fooled – blank just means it will show the default text “SharePoint”
Change the header text
$web = Get-SPWebApplication “https://CentralAdminURL”
$web.SuiteNavBrandingText = “Company Name”
Refresh your browser for the Web Application URL and you should see the changes. You may need to re-open your browser.