Can't Change the Font Size for the Header

I've looked this up already on forums but the typical answers aren't working. All I am wanting to do is change the font size on the header and nothing is working. I've done format > style > text style > header and nada. Anyone else run into this problem?

I know a possible cause of the problem-- I had to download the musescore file from the website because I didn't have the file of the song I was going to work on on my computer, I wrote it several years ago. Everything else is working, but is it possible that something is wrong because I downloaded an older version? Any tips would be appreciated. Thanks.