Replace missed values with NA [duplicate]

I have a dataset but some of the values has ? rather than a valid value.

I would like to replace the ? with real NA so I can omit them easily omit them with

na.rm = TRUE 

to do this I used openrefine, and

Edit cell -> Transform ->value.replace("?","NA")

Although, it changes the ? To NA but the type of NA is string, so it is not useful

How can I convert it too a real NA?