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 just click one action button and execute multiple actions, for example, executing Create Delivery Note (Convert records) and SO Complete (Update values) simultaneously, you can refer to 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 configure. 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.