Excel formula/function help

[WendyBW]

In spreadsheet A, I have a column with customer numbers (A) and a column of customer names (F). 

In spreadsheet B, I have a column with customer numbers (BX).  I want to replace the corresponding customer numbers in BX with the customer names from spreadsheet A, column F.

I've tried Index/Match and other functions and am not getting anywhere close to getting it right.

Any suggestions?

[#2]

By formulas you can't take the value from the cell (in BX), make some calculations and return the result into the same cell.

By INDEX/MATCH you may return names into another column and hide BX.