Even if you succeed in relinking it to the epic Launcher (by clicking UE4Editor.exe or UnrealEditor.exe, according to the version), it will be displayed as “Source build at %yourPath%”, and you won’t be able to download plugins or update it from the marketplace (content packs should work normally).
The best thing you can do is downloading it again (I wish there was a better way, I have like 5 engine versions installed and every time I format it’s a pain to reinstall them all)