Open and edit html files

I have over 400 HTML files in a directory on a drive that I need to open, find a few different custom tags formatted for a previous email service and replace the tag with the format for the new service and then save the changes.

The tags are place holders for data and I need to find something like this  |%firstname%| and replace it with something like this [%first_name%]

 

I know how to iterate through all the files in the directory.

I was a developer but it's been10 years at least, however, I did write in vb.net and some c#(Today it's almost all T_SQL for me at work) 

But I'm not sure of the project type that would be best 

 

I have tried a few things but honestly, don't have any code that worth including at this point

 

All and any help will be sincerely appreciated