A while ago the fan started to make a hell of a noise, i opened the tower up and fuck me there was some ammount of dust/shite in there

I hoovered it and it was fine for about 2 years but it has started to get noisy again, a quick hoover or a bang sorts it

Is it hard to replace the fans, i spotted 2 when i took the side casing off, one looks easy a wee 2 - 3" one but the another at the top looks more complicated to change

Is it hard or expensive to do.