If you've used the popular plugin A&M Nav (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 A&M Nav plugin heading to automatically migrate your A&M Nav's to Navigation.
If all went well, you should have your navigations appear in the Navigation page in your Control Panel.