You have to apply at the Embassy of the Schengen member state in whose territory your main destination is located.

Schengen States are : Germany, Austria, Belgium, Croatia, Denmark, Estonia, Finland, France, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxemburg, Malta, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland, The Czech Republic, The Netherlands,

The maximum duration of stay in the Schengen area is 90 days within 180 days. If you are holder of a Schengen Visa you are allowed to visit all Schengen countries. However, your main destination has to be the Schengen country you applied the visa for.

You can travel again in 3 months. You can calculate your stay and the remaining days here: https://home-affairs.ec.europa.eu/policies/schengen-borders-and-visa/border-crossing/short-stay-visa-calculator_en

Please present your passport and a letter stating your wish to cancel your visa at the embassy. Please note our opening hours.

Yes, you can. However your main destination should be the Schengen Country who issued the visa

The processing time of a Schengen Visa application is 14 days.