By design, there is currently no option to update the MTU of an existing network pool through the SDDC Manager UI. Core network pool parameters, such as the MTU, are immutable once the pool has been created or is in use.
VCF 4.5.2
VCF 5.x
To update the MTU size for an existing network pool in SDDC Manager, the changes must be made directly within the SDDC Manager database. Hence please create a ticket with Broadcom Support.