Putting in "H2" tag manually is really the only option. You can't do a find and replace or it will change all the first level TOC entries to second level as well.
Hit the generate TOC button and have a look. What's ticked is in your TOC. Clicking "Cancel" will get you back out without making any changes. It displays in a route tree format like windows explorer. Your sub headings MIGHT already be "H2", then it is just a caase of checking and unchecking the boxes as required.
I think you might have to put on a pot of coffee and roll your sleves up my friend
