How to replace an underscore character with a blank space

First off, thanks for the all of the spot on and timely help. I just dug through the forums for this one but can't seem to find this specific case. I have many files that have underscores between different fields which prevent these files from being renamed with this wonderful utility. An example is FIELD1_-FIELD2-_FIELD3. The desired result for this example is: FIELD1 - FIELD2 - FIELD3

THANK YOU IN ADVANCE!

You replace one string constant with another, so use a simple replace action for _FILENAME:
Search string: _
Replace string:
(if you look closely, you see the space character ;-))

Or, if you want to make it absolutely sure:
Search -
Replace: -
(blank-hyphen-blank)

That makes complete sense. I just don't know where to do this, Convert, Actions and if Actions do I need to make an Action group? What kind?

Yes, it is an action.
An action group is not really necessary (and only defined by a collection of actions under one name).
If this is a one-off for cleaning up the collection and not a regular job, then you can create an Action (Quick).

Okay that worked perfectly as well. The last task for this mass file clean up, if I want to add a space on each side of a "-", can I do that? so if I have a filename: xxx-xxx-xxx can I rename it to xxx - xxx - xxx?

You can easily apply a similar action like for the underscore for _FILENAME.
Search: -
Replace: -
(blank-hyphen-blank)

This may lead to double blanks in files that already have at least one blank in front or behind the hyphen.
So I would run a second replace action that replaces a double blank with a single blank.
I think, you know by now how.

Thank you again sir!