Do you only need to change the knob/handle itself - and leave the lock/latch mechanism in the door itself in situ?
If this is the case, the advice received thus far, may be more than required.
Bend down and look under the handle, but just a bit in towards the door. you should see a screw, but will probably be in a fact a hexagonal "hole", which needs a certain type of tool to undo called an allen key (Inbusschlüssel in DE/clé allen in FR):
Once you can undo this pull both handles/knobs from both sides. The handle that you have undone the screw will come away alone and the other will come away with a square spindle.
You need to replace with handle/knob of your choice - one of which is attached to the spindle and pushed through the square hole in the door latching mechanism. Once through put the other handle on, push both handles together and tighten the screw.
N.B. It is usual that a screw is on both sides by each handle, and both should be tightened.