This article explains you how to override a view in AcyMailing. This is useful when you need to modify the structure of a page. If you simply want to hide some options, we highly recommend to use CSS or ACL instead.
If something isn't working on the overrided page after an update, remove your override and do it again to make it compatible with the new Acy version.
For Joomla 3, go in Extensions => Templates => Templates then select the Back-end or Front-end template depending on the page you want to override.
On this second page, click on the "Create Overrides" tab, then on "com_acym" then click on the view you want to override.
Once done a new folder will be created on your site, you will then be able to access it and modify those files via FTP to change the way AcyMailing is displayed.
On Joomla 4, go in System => Templates => Site/Administrator templates then follow the same steps: