How do I find and replace a character in filenames in Windows 7 using Explorer?

vbScript should do it for you. Create a file called "rename_underscores.vbs" containing the following.

Set objFso = CreateObject("Scripting.FileSystemObject")
Set Folder = objFSO.GetFolder("c:\test\")

For Each File In Folder.Files
    sNewFile = File.Name
    sNewFile = Replace(sNewFile,"_"," ")
    if (sNewFile<>File.Name) then 
        File.Move(File.ParentFolder+"\"+sNewFile)
    end if

Next

Make sure the folder name is correct. (In the example, I've used c:\test) And then double click your file to do the renaming.

As others have mentioned, there is no way to do exactly what you want without using a script or batch file. For instance, in PowerShell you can do what you want quite easily:

cd "C:\Users\MyName\Documents\MyDirectory"
Dir | Rename-Item –NewName { $_.name –replace "_"," " }

Just be sure to replace "C:\Users\MyName\Documents\MyDirectory" with the path to your directory.

For more detail and other options, including some things you can do using just Explorer, see here: http://www.howtogeek.com/111859/how-to-batch-rename-files-in-windows-4-ways-to-rename-multiple-files/

I recommend using a 3rd party tool like Bulk Rename Utility. It's free and it has an installer less than 1 MB.