dropdown tool

[ShreyaValte]

I want the output which I am getting in excel file as a output of formula tool.
I just need to append the column K,F,C any one of them in column Temp, I am choosing It from dropdown.I am getting the value in output but unable to get it as a output of formula tool.

[KGT]

It's hard to tell if this is the only problem based off screenshots, but you are replacing [F] with F. This will put a string in rather than a column name. Either just replace the F, or replace the entire expression with [F].

[ShreyaValte]

actually I am getting the correct output in output file, but output from formula tool is not getting replaces 

[TUSHAR050392]

If I understand it correctly, you want to replace the value of Temp by the selection that you do in the interface tool. If you select F then Temp should have the value of F column and so on. I think what you are missing in square brackets in your selection because you are trying to update [F] by just F or K or C so it is not recognizing it as a column. Hope this helps.