Why might you want to do this?  The flat fixture is much simpler to use and understand, and it's faster!

I'm making a new domain and want to use the flat fixture

We got you covered,  all new domain will use the flat fixture by default.

I have an existing domain that I want to upgrade to the flat fixture

Go to "Project Settings" then on the left, under "Pre-Release Features" click "Location Fixture".  If you don't see this link, you're probably already on the flat fixture.  That page lets you configure which version to send to the phone.  If the flat fixture is enabled here, your forms will automatically try to reference the flat fixture in new builds of your app.

We expect that the migration will go something like this:

1. Starting off, you may have an existing app which references the old fixture, and this app may have active users.  The idea is to be able to migrate your app without disrupting active users.
2. Go to the aforementioned "Location Fixture" page and check "Sync the flat location fixture".  Do not uncheck the other box yet.
3. Begin converting your app to use the flat fixture as described here.  You can deploy the new app versions to a phone and test them out, and your existing users won't be affected (As usual, don't star any new builds until it's working so your users don't accidentally update).
4. Once you're confident that it's working, star the new app version and get your user to update.
5. Once none of your users are using the old app version, you can uncheck "Sync the hierarchical location fixture" to reduce the size of the restore payload that must be synced to the phones.

I'm making a new domain and want to use the old hierarchical fixture

Are you sure?  This is highly discouraged.  If you do need to do it (for instance, to use an app copied from an older domain), please contact Dimagi support for assistance.