If your template has a file "templates/system/html/modules.php" you are ok. Else copy "modules.php" from "templates/system/html" to "templates/yourtemplate/html".
Open "modules.php" in "templates/yourtemplate/html" using your favourite editor or through the administrators template files editor.
Attention! If you copied the system/html/modules.php you also need to rename all the
to something else to avoid errors.
I myself named the functions
I used "_alt" as a suffix as I use it in a prefix later on.
Now save your altered "modules.php". Also make these changes to your template index.php file. Look for all styles and make the changes
<jdoc:include type="modules" name="left" style="html5_alt" />