This is possible only in MultiChain 2.0, for which alpha 3 is available.
If you decide to transition, please note that you can't go back again, and all nodes would have to run on MultiChain 2.0 alpha. You would first need to upgrade the blockchain protocol to version 20002 or 20003, then create a second upgrade event to change the target-block-time.
The basic process for creating and approving upgrades is described here:
https://www.multichain.com/developers/upgrading-nodes-chains/
See also the notes on the MultiChain 2.0 preview page about other upgrading constraints (search for create upgrade on that page):
https://www.multichain.com/developers/multichain-2-0-preview-releases/