let (one) know
To tell or inform one (of or about something). Be sure to let me know if anything changes between now and then. I let the boss know about the development yesterday.
let someone know (about something)
to tell someone something; to inform someone of something. Please let me know about it soon. Will you be coming to the picnic? Please let me know.
let somebody ˈknow
tell somebody about something: I don’t know if I can come, but I’ll let you know soon.Let me know how I can help.