The "Regenerate/reset" function is most usually used to make the permalinks adhere to the default formats specified in the permastructures settings. This functionality may be very helpful if you need to quickly update the permalinks of certain post types or taxonomies items.
Custom permalinks are stored statically in the plugin and are not automatically updated by default. This gives users complete control over when and how their permalinks are modified, which helps to avoid frequent permalink changes.
This implies that if a user changes the permalink format ("Permastructure") for a specific post type or taxonomy, the old permalinks will stay unchanged. Instead, the new format will only be used for new items. To apply the new format to older items, users must use the "Regenerate/reset" function.
This approach allows users complete control over their site's URL structure, allowing them to make modifications as necessary. However, it is important to be careful when changing permalinks that have already been indexed because it could affect your site's SEO. If you are unsure about this or if you need extra permalink SEO tips please read this post.
How to regenerate the custom permalinks?
To open "Regenerate/reset" tool and regenerate existing permalinks, go to "Tools -> Permalink Manager -> Tools" section. Please pick "Regenerate custom permalinks" from the first dropdown ("Mode"). Then, depending on the content type, pick either "Post types" or "Taxonomies" in the "Select content types" section.
Depending on your requirements, you may run the tool for different post types ("Select post types") or taxonomies ("Select taxonomies") at the same time. Using the "Select IDs" field, you may also target just particular posts or terms. Finally, to start the process click the "Regenerate" button.
If everything went properly, you should see a list of updated permalinks displayed below the form.
How does the "Regenerate/Reset" tool work?
To put it simply, whenever the URI is regenerated, Permalink Manager will replace the currently used canonical permalink (see "Custom permalink" below) with its default version based on current Permastructure settings (indicated with "Default custom permalink" field).

You can prevent URIs that have been modified manually from being overwritten by excluding them in URI Editor. For further information, please see the next section of this article.
How to exclude custom permalinks from "Regenerate/reset" tool?
You may exclude certain permalinks from processing in the "Regenerate/reset" tool if you manually modified them to avoid accidental overwriting. In order to do this, under the "Permalink update" section of the URI Editor, choose "Don't auto-update "Custom permalink" and exclude from the "Regenerate/reset" tool".
How to automatically update single permalinks?
If you only need to reset a few permalinks, you may do it manually in URI Editor by clicking the "Restore Default URI" button.
The "Restore Default URI" button in the individual post editor and the "Regenerate/Reset" tool both perform the same thing, which is to change the "Custom permalink" value to the one specified in "Default custom permalink".

You can find more detailed instruction on this here.
How to regenerate native slugs?
The procedure is similar to the "Regenerate custom permalinks" mode. The new native slugs will be created from a single post/term titles. In this mode, you will be unable to restore the original slugs unless you have a database backup.