Changes of visa validity dates are not possible after the visa has been issued.
The procedure for a re-issuance of your visa with different validity dates or other data is as follows:

• Set a new appointment via the appointment system of the Visa Section
• Arrive in time for your appointment
• Submit a new visa application with the corrected dates
• Submit any documents (e.g. an updated travel health insurance) that were submitted with the original visa application and do not cover the additional days for stay in the Schengen-Area
• Pay the full visa fee

The processing time should be 2 working days.
Kindly be aware of the following when applying for a Schengen-Visa:
The Embassy issues visas with validity dates according to the data provided by the applicant in the visa application form.
The application form is the document that is finally legally binding in terms of the application data that is given by the applicant.
Please make sure that the date of entry to the Schengen-Area (point no. 30 in the application form) and the date of departure from the Schengen-Area (no. 31) correspond with the duration of stay (no. 25)