Hi Robin, Is this person who said that a native English speaker? Because we usually say ASAP. Sooner is at their earliest convenience.

Thanks Diana!

yes, it is a normal English expression, but not in the context of "I will update" It does not sound natural, because it means: without to much delay. I will update without to much delay is not incorrect but does not sound natural, neither does , I will update sooner rather than later. Maybe if you said, I would rather update sooner than later. I would rather go shopping sooner than later. English is a very complicated language and to make it more complicated there are many ways of speaking it depending on where you live in the U.S. Remember this is just my opinion and I respect others as well.

Oh I see! Okay Su.Ki! Thank you very much for your help!

I'm having to reply to this as a comment because you closed your question after only one reply! In fact, "sooner rather than later" is a perfectly normal adverbial expression which native English speakers use. It means "before too long", so it isn't as urgent as "as soon as possible". Remember that not everyone is aware of every expression in the English language, so it's worth waiting for a few answers .