I am completely new to Robohelp and very confused.
I need to find every line in the .xpj that uses the 'Letter1' or 'Letter2' style and change it to the 'Normal' style. Is there a way to do some kind of Ctrl+F search for lines that use a specific style? Or is there an easier way to do this rather than manually go through every page and topic and check every line?
Firstly that information is not held in the XPJ file, it is held in the HTML of the topics. Secondly, your version is very old and likely encounter problems with the main browsers, if not all browsers, if you are generating webhelp.
The only way to do that is using Search All Files and going through the code to find all the tags and convert them. You also need to do that with the closing tags. It is not something I would recommend to someone new to RoboHelp as there is too much chance of getting something wrong. Maybe you are new to RoboHelp but nonetheless a competent PC user and if that is the case, you may feel comfortable about doing this. Do create a backup first though.