You replace the Bernette B44 - B48 lower blade when the moving blade has become blunt!
Replace the movable upper blade or lower blade yourself ?
There are always two blades in u overlock machine these two blades work like scissors.

Always unplug the machine first
1. Loosen the screw and take the knife out of the knife holder
2. Insert the new blade into the blade holder and secure it again with the bolt.
3. Check that the cutting edges of the lower and upper blade overlap by about 0.5mm

* Replace the upper blade as well! We recommend replacing both the lower and upper blade to prevent the new lock blade from being damaged by the old blade.