If you've used the popular plugin Navee (opens new window) for Craft 2, there's an easy way to migrate your navigations to the Navigation plugin. This is only applicable for Craft 2 to Craft 3 upgrades.
After migrating your Craft 2 install to Craft 3, and have installed Navigation, visit Settings → Navigation page in your Control Panel. Click on the Migrate button under the Migrate Navee plugin heading to automatically migrate your Navee's to Navigation.
If all went well, you should have your navigations appear in the Navigation page in your Control Panel.