According to the Microsoft Docs page, the syntax is:
ALTER MASTER KEY REGENERATE WITH ENCRYPTION BY PASSWORD = & # 39; password & # 39 ;;
From that document page:
The REGENER option recreates the master key of the database and all the keys it protects. The keys are decrypted first with the previous master key and then encrypted with the new master key. This resource-intensive operation must be scheduled during a period of low demand, unless the master key has been compromised.
However, make sure you understand the ramifications of changing the key and be sure to make a backup of the new key.
Check my recent post for more details.