This blog is a very simple two-step process to upgrading the RouterBOOT firmware from both Winbox and the New Terminal windows.

Navigate to System > Routerboard > Upgrade.
Select "Yes" to Upgrade.
Navigate to System > Reboot.
Select "Yes" to Reboot

New Terminal:
[admin@MikroTik] > system routerboard print
routerboard: yes
model: 433
current-firmware: 3.03
upgrade-firmware: 3.07
[admin@MikroTik] > system routerboard upgrade
Do you really want to upgrade firmware? [y/n]
firmware upgraded successfully, please reboot for changes to take effect!
[admin@MikroTik] > system reboot
Reboot, yes? [y/N]:
system will reboot shortly