Perhaps you don't understand - it is not text but an MPEG 2 video created from a bitmap. You can't just "change the text". You need to feed a new image to replace the old one.
The whole process, assuming you already have a modified bitmap, shouldn't take more than a few minutes. The muxman authoring will happen in a flash (just feed it the BMP, no audio or subs), the replace with still no more than a few seconds and then you can export modified files (which given they are menus is very quick). Then, move the modified files back into your project folder.