Just replace the bad fans on your case.
Most common way how case fan is connected to a case is by 4 screws, one at each corner of the fan. Just unscrew those to get the fan out of the case. Also remove the fan power cable from whichever socket it is plugged to to remove the fan completely. Keep in mind where the fan power cable was connected since replacement fan's power cable connects to the same fan connector.
If fans are held in place with screws and depending from which side the screw heads are (inside or outside), you may need to remove the front/top grille to gain access to the screws when screw heads are on the outside. When screw heads are in the inside, then removing top fan is easy. But to remove front fan, you need to remove HDD cages first, so you can get the access to the front fan.
Here's a video review of 600T that shows how to remove the front/top grilles from 600T,
youtube:
https://www.youtube.com/watch?v=A3VG92Of2FU
As far as replacement fans go, Bitfenix Spectre Pro series fans have good airflow and they also come with LED illumination,
specs: https://www.bitfenix.com/global/en/products/accessories/spectre-pro-led/
200mm pcpp: https://pcpartpicker.com/products/compare/zBkD4D,Y4ckcf,nZNp99,tx8Zxr/
120mm pcpp: https://pcpartpicker.com/products/compare/B8DwrH,ZsDwrH,f6JwrH,svPfrH/