Start on the driver's side. Gently lift the wiper arm on your 135i and release the wiper blade. Blades are typically held in place with a small button to push or tab to pull. Once loosened, most blades will slide right off the wiper arm with some gentle wiggling.
Position the new blade
Place the new blade near the connection point on the wiper arm. If the connector on the arm is a hook, put the wiper blade adapter at the open end of the hook. If it's a bayonet, look for a slot on the wiper blade connector about the size of the bayonet and position the blade in front of it. If it's a side pin, make sure the correct adapter is on the wiper blade line up the hole on the side of the wiper with the pin.
Attach the blade
Push the blade on to the wiper arm. You'll hear a click when it locks into position.
You're all finished with the driver's side. Repeat the process on the passenger side of your 135i.