Unless someone is on Medi-Cal or relocates to a different region mid-year, you cannot select a change in your plan coverage until the next Medicare Annual Enrollment (October 15 thru December 7) to be effective 1/1/2018.
Part of the enrollment packet you received included a prescription drug formulary. The formulary lists the prescription drugs covered by your current plan. I recommend that anyone on Medicare take their drug plan formulary with them to all medical appointments or have it by the phone if they are expecting a call from their doctor so that they can confirm with the doctor whether or not the drug s/he is suggesting is covered by your plan as well as if it might be a much higher tier/more expensive drug.
Contact your prescribing physician to let him/her know you need an alternative prescription drug that will be covered by your current formulary.