This is the safest method for most administrators.
If you cannot physically access the PBX, you can use PCPro over a serial connection (or IP if the system is still responding). nec sl1000 firmware upgrade
Warning: Over IP, this is risky. If your network drops a single packet during the flash write, the CPU can become bricked. Always prefer USB or a direct serial cable. This is the safest method for most administrators
The golden rule of PBX administration: No one upgrades firmware on a Friday afternoon. Always schedule the upgrade during a maintenance window with a rollback plan. Warning: Over IP, this is risky