Peavey Ultra Help with Tubes

[ChlorineRR4]

well, i think my tubes blew in my Peavey Ultra 112 combo

it says it has 4 tubes, but i only see 2 large ones(?)

are these something i can replace myself by just slapping 2 new tubes in?
if so what kinda tubes do i need to ask for when i hit the music store?
will getting better tubes improve the amps sound?

or should i let a pro do it?

thanx for any help!