how to replace a character using mod_rewrite?

I replaced the spaces in old html filenames that contained spaces with underscores. Now, to make sure that visitors who may be following old links get to the right place, I would like to use mod_rewite to redirect them to the new filename.

For example, if a visitor asks for

file%20name.html they should be directed to file_name.html

I would be grateful if anyone could explain how I need to define this rule in my .htaccess.

 

 

 

 

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