Each router has different software, so we cannot tell you specifically how to change the MTU on your router, but the setting is typically under Advanced Settings or a similar item in the menu of configuration screens.
Any network interface attached to a network can receive every bit of traffic. However, most are configured not to do so. The software packet sniffer adopts promiscuous mode to change this configuration. This enables the interface or adapter to pass all network traffic up the stack, regardless of the packets' destination addresses.
There are two ways to change the default port: the Winbox software itself and the other through the Mikrotik console (Terminal). It does not matter which way you do it, and it depends on which method you wish to use.
In those cases it might be better to keep the routing table unchanged and hope that the hardware in the upstream provider edge router will keep forwarding packets while the software figures out how to recover from a crash. The potential outage might be shorter than the brownout caused by changing the best path for a million prefixes twice (assuming the primary upstream router crashed). 2b1af7f3a8