1. First, make sure that the trimmer is turned off to ensure safe operation.
2. Look for the cutter head release button on the trimmer. This is usually a small lock/release button on the side or back of the trimmer.
3. Press the cutter head release button to unlock and remove the cutter head from the trimmer. Usually, you just need to gently press the button with your fingers to pull out the cutter head.
4. Ensure that the new cutter head is compatible with Braun 9 series sideburns trimmer. When buying a new cutter head, you can consult the sales staff or consult the product manual.
5. Align the new cutter head with the groove on the trimmer and ensure that the cutter head is inserted correctly.
6. After fully inserting the cutter head, press the cutter head release button to ensure that the cutter head is safely locked on the trimmer.
If you are still not sure how to replace the cutter head, I suggest you consult the instruction manual of Braun 9 series sideburns trimmer or contact Braun customer service for more detailed instructions.