For just one filename i would use the explorer.
if there are more files to be renamed, I am not quite sure whether a search/replace on the pseudo variable filename works.
If it does not:
format a custom tag with the name e.g. "newfilename" with %_filename%.
replace all blanks in "newfilenmae" with nothing with a simple replace action.
copy the value of newfilename into the pseudo tag %_filename%.
the FAQ
/t/967/1
shows you how to copy values from one tag to another.