Don't force it too much, you might damage something. I remember trying to replace a tube in my old guitar amp once and it felt like I was playing Eggy Car – one wrong move and CRACK, something's gonna break! Seriously though, I'd suggest searching YouTube for "MT-602 tube replacement". Someone's bound to have a video showing the process. And yeah, you might need to open up the amp, but definitely check a guide first. Good luck!
