If you can obtain the firmware file from a trusted source, here is how a standard firmware update process works (these steps are general, based on common router interfaces):
Example search string: site:prolink2u.com IBWRB303N firmware OR "IBWRB303N" filetype:bin 300m wirelessn broadband router ibwrb303n firmware
Never update router firmware over Wi-Fi. If the wireless connection drops for a split second during the flash process, the router will break. If you can obtain the firmware file from
Try clearing your browser cache or switching to an Incognito/Private window. The Firmware File Is Rejected 300m wirelessn broadband router ibwrb303n firmware
The IB-WRB303N is an older 802.11n model. If you are experiencing significant speed issues and the firmware update does not help, consider upgrading to an 802.11ac or 802.11ax (Wi-Fi 6) router, as the 300Mbps cap may be limiting your modern internet speed.