How to free SQL Server memory?

Stop/start the service, nothing else will release the memory back to the OS.

Obviously not something you'd ever want to do with an operational server but perfectly reasonable for a local sandbox. With 3 different instances on my laptop, its the only viable way.

Edit following @Nick's comment.

As James noted, if it reserves more than the minimum you specify, it should in theory release it when it doesn't need it and there is memory pressure on the machine.

What's your definition of "doesn't need it"? Once the buffer pool has been filled, it isn't going to drop pages out until they become disfavoured by other pages being brought in. If you want the memory back immediately, as is the case for @jrara, you've got to stop/start.

DBCC FREESYSTEMCACHE ('ALL') 
DBCC FREESESSIONCACHE
DBCC FREEPROCCACHE 

will free up the pool .

https://msdn.microsoft.com/en-us/library/ms178529.aspx

Remarks

Executing DBCC FREESYSTEMCACHE clears the plan cache for the instance of SQL Server. Clearing the plan cache causes a recompilation of all subsequent execution plans and can cause a sudden, temporary decrease in query performance. For each cleared cachstore in the plan cache, the SQL Server error log will contain the following informational message: "SQL Server has encountered %d occurrence(s) of cachestore flush for the '%s' cachestore (part of plan cache) due to 'DBCC FREEPROCCACHE' or 'DBCC FREESYSTEMCACHE' operations." This message is logged every five minutes as long as the cache is flushed within that time interval.

It will give up the memory it is not using automatically as other apps try to commit it.