How do I change the presser foot on an elna mini sewing machine?

I dodn't know your model really well but it will be one of the following:
a clip on foot that you just pull down on to remove,
a clip on foot with a button at the back of the pressure foot holder - press the button to release the foot
a clip on foot with a little lever at the back of the pressure foot holder - lift the lever to release the foot
If it is a clip on foot, then you just need to raise the pressure foot lever, put your fingers onto the pressure foot and press downwards, it should unclip.

To put one on, place the pressure foot onto the needle plate under the pressure foot bar and lower the pressure foot bar down onto the foot, lining up the base of the foot holder with the little bar on the foot, it should click into place.