Mass Find and Replace?

Greetings,

I am pretty much an Excel noob, and I'm trying to do a mass find and replace with different starting and ending values. I have data in number form, and I have a 'translation' worksheet. So in sheet 1 I have:
City (column)
12345
12345
12346
12356

with a whole bunch of additional data

While in sheet 2 I have (columns 1 and 2)
City Ref #
New York 12345
Los Angeles 12346

What I would like to do is something like
if Sheet 1(A1) = Sheet 2(B1) replace with Sheet 2(A1), repeated across all Sheet 2 B values.

Is there a way I can do this?

Also - is there a good primer on these types of things, so I can figure it out for myself in the future?

Many thanks everyone.

Sorry, which formula?

It works, thank you! (I clicked the star)