I've been given a Schengen visa valid for 1 month. Unfortunately, I won't be able to travel during the dates I've specified in my application. What should I do? Do I just not use the visa? Or should I let the embassy know so they can cancel it?
You can write to the issuing post and ask them to revoke your Schengen visa. It can be something simple like...
Due to a change in circumstances I will not be using my Schengen visa (Visa #XXXXX, Passport #YYYY). Accordingly, please revoke it. I will make a fresh application at a later date.
There is no prejudice attached to a voluntary revocation; in fact, they like it when people do that. When you apply again, they will see it in the database and your following procedures will be counted in your favour. But you can also do nothing and let the visa expire.
Either option is good.