A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
You can not use a formula in a cell to replace the value based on that cell value itself.
You can however enter this in C1, assuming you have your data in Col A and B
=IF(A2="NA",B2,A2)
and copy down.
You can then copy values in Col C and PASTE SPECIAL -> AS VALUES over values in Col A
Result