How to change door rubber on miele premier 300, can the front of machine be opened allowing to change the door rubber

I'm assuming you mean the flexible rubber between the door and the drum. Can it be replaced? The short answer is no! However...
I recently repaired a door rubber with a tear that caused the water to flood onto the floor. Here's how:
Yes, you can open the front to get to the door rubber however the rubber seal is glued to both the door and the inner drum lip.
You may have to tilt the machine on it's back to remove three screws from the underside of the front lip. (I forget if you need to take the water pump cover out as well but it wouldn't hurt to do so.) This allows the front panel (including door) to drop down a quarter inch or so and be eased out a little.
This allows *better* access to the drum rubber. To repair a tear - really the only reason you would want to do this - you need glue and some tape.
The door rubber is silicone and will stick with normal 'super glue'. However, bear in mind that these machines can do a wash up to 95C, which is hot enough to soften superglue.
I found a new version of superglue called Repair Extreme by Pattex that copes with temperatures up to 120C. It sets in about 5 minutes.
The glue needs to be flexible. Do not use epoxy resin.
I gave the glue some form by sticking a 10-15cm piece of duct tape - the silver shiny plastic tape plumbers use, readily available from any hardware and most supermarkets - to the *back* of the drum rubber. It's a tight squeeze to get your hands in there but it can be done.
Take the tension off the drum rubber by moving the front panel roughly back in place.
Then squeeze a generous amount of glue along the tear (about 3cm long on the bottom edge; a typical problem) to completely whet the edges to be joined.
Apply a little pressure to the location of the tear for a few minutes to help the glue hold fast. Leave the glue to set for at least half an hour before reassembly.
If you can source a replacement drum rubber the old one can basically be removed by force. A good cleaning and then a suitable glue could stick a new unit in place. However, if this is required, seriously consider buying a new unit. It is difficult to make a good seal that will not leak and may not be worth the effort in the first place.