Microsoft looks to be giving a new firmware update to their budget Windows 10 Mobile device the Lumia 550. Firmware version 15503.21007 is now available through the Windows Device Recovery Tool, which is a bump from the shipping version of 15461.48007. The firmware update mirrors the one Microsoft rolled out for the Lumia 950 and Lumia 950 XL at the end of 2015.
Unfortunately, the firmware is not attainable through an over-the-air (OTA) update just yet. That means if you want it now you have to use the Windows Device Recovery Tool for PC to wipe your device and take the update. That is a lot of work for an update that as of now has no changelog, so unless you're already having serious issues with your phone and don't mind a hard reset, we would advise you that you skip this method and wait for the OTA update.
The OS version for the Lumia 550 stays the same through the WDRT (build 10586.29), but users could enroll in the Insider program to get the latest version if they wish.
Microsoft has not yet announced the new firmware update for the Lumia 550. However, putting the backup software in the recovery tool is the first step before they push it out to phones through an over-the-air update. Microsoft is believed to be releasing new OS updates tomorrow, and this new firmware could be announced with that push or sometime soon after.
We're currently flashing a Lumia 550 with the new firmware and will post if we find anything noteworthy. Nonetheless, this update is likely to fix bugs and optimize performance.