Hello,
First of all thanks for choosing our Theme, we are glad to be you in the WoodMart WordPress family :-)
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To edit the categories sub menu you need to edit the HTML Block used in the on hover section in the categories menu items from Appearance >> Menus.
Please Go to Appearance >> Menus >> Edit categories menu >> Edit item >> Html block as drop down.
Screenshot for Clarification: https://ibb.co/5FysmXk
If you want to ask anything else then provide with some relevant screenshots of the specific area to check it myself and help you out accordingly.
Best Regards.