Hi there! To replace a sidebar on a specific page, go to the page you want to change the sidebar on and use the sidebar module to select a custom sidebar.

If the sidebar module is not showing check the screen options to make sure the sidebars module is active.

If you are not finding a sidebar option, make sure your theme has sidebars on pages. Custom Sidebars will only replace existing sidebar areas. It will not create new widget areas.
Let me know how it goes!
There is not an option for the main sidebar, but for all other types: blog, post, arbitrary and footer plus a couple of custom ones that are included in our theme.
Please check to see if the “Allow this sidebar to be replaced” is checked for the main sidebar on your pages.

Thank you for using Custom Sidebars. I am going to mark this topic resolved because we haven’t heard from you in awhile. If this issue was not resolved we can reopen it. Thanks again!