How can I make this exercise a little bit more challenging or a little bit more easier for the clients exercises such as squats shoulder press lunges and kettlebell swings and plank for example.

To make the exercises more challenging:

1. Increase the weight of the kettlebell used for swings.
2. Add an overhead press to the squat exercise.
3. Incorporate jump squats or plyometric lunges for an added cardiovascular challenge.
4. Increase the duration of the plank hold.
5. Add in a balance element to the exercises by performing them on an unstable surface such as a BOSU ball or balance board.

To make the exercises easier:
1. Use a lighter kettlebell for the swings.
2. Perform the shoulder press sitting down to reduce the stability required.
3. Use a resistance band for assistance with lunges or squats.
4. Perform a modified plank on your knees or start with a shorter plank hold duration.
5. Reduce the range of motion in the exercises, such as performing shallow lunges or partial squats.