On sheet design page, you can add custom action buttons to these four features you'll find under Form Tools > Custom buttons section:
If you want to execute multiple actions through a single action button click, for example, simultaneously executing Create Delivery Note (Convert records) and SO Complete (Update values), you can create Combine Buttons.
You can edit existing custom buttons from the design page by going to Custom Buttons section and clicking on the action button name you'd like to edit. Save changes after you finish.
For more details on additional settings for action buttons, please refer to this article.
To remove action buttons, please refer to this article.