Please try the methods below and in this order:
Method 1: Most of the time, it will work best to get a fingernail behind the sleeve and pull the sleeve off the stem. "Walk" the sleeve off the stem by moving the fingernail from one side of the stem to the other. While pulling upward with the fingernails, rotate the earphone with the other hand.
(While holding the earphone, twist and pull the earbud off)
Method 2: (If your finger slips) Use a shirt, a towel, or paper towel to securely grasp the sleeve. Twist and pull the sleeve away from the nozzle while gently rocking the sleeve from left to right.
Method 3: Place the earphones in the freezer overnight. The plastic will shrink slightly and break the adhesion between the sleeve and the nozzle. The cold temperature will not damage the earphones. Be careful: Do not break off the earphone nozzle.
Method 4: This method will ruin the flexible sleeve. Use a staple remover. Clamp it around the flexible sleeve and gently pull upward. The earphone stem is small enough to fit the gap in the middle of the staple remover jaws.
Try method 1 first with gloves (with good grip). Method 2 and Method 3 have a risk of breaking the earphone nozzle.