should I clean my fans manually or replace them?

Cleaning is quite easy! Get a small soft pain brush and a can of can'ed air so you can blow out the dust & debris.

I would follow these two guides as its better if you remove them:

• MacBook Pro 15" Retina Display Mitte 2012 Lüfter links austauschen
• MacBook Pro 15" Retina Display Mitte 2012 Lüfter rechts austauschen

Carefully scrub the blades as well as the heatsink fin areas. I blow the air outward across the fins and a few short blasts on the fans. You don't want to spin them up, I often carefully wedge a toothpick to hold the blades from moving. Don't for get the give the logic board a quick wipe as well with the brush and then blow off what you've loosened up.