On this page, you will find how to make template edits in plugins.
Here is how to make a basic template edit. For this example, we are going to edit the index template and add the variable {$randomvar} after the board statistics.
function pluginname_activate() { global $db; require MYBB_ROOT.'/inc/adminfunctions_templates.php'; find_replace_templatesets( "index", '#'.preg_quote('{$boardstats}').'#', '{$boardstats}{$randomvar}' ); }
To reverse the template edit do this:
function pluginname_deactivate() { global $db; require MYBB_ROOT.'/inc/adminfunctions_templates.php'; find_replace_templatesets( "index", '#'.preg_quote('{$randomvar}').'#', '' ); }