Kapow Device Automation Replace Spaces with underscores

Hi friends,

Can i know how to replace text spaces with underscores??

(kapow Document.xls) this is my original text. It's extracted value. I need to convert that document name to (kapow_Document.xls).
I used the replace text expression like this. 
replaceText(FileName, "_", " ")

FileName- the variable.

Above Expression is not working with device automation assign variable step. Can I know what is the correct format of this??

Thank you.

Friends, 
I found the solution for this.
replaceAll($extract, " ", "_")
$extract  is the variable that used for assign the extarcted value.