Rename it to 550.PBP, drop it into the UPDATE folder included in the 5.50 Gen-A Installer archive, and drop that UPDATE folder in \PSP\GAME (like any other bit of homebrew).
The 5.50 Gen-A installer will begin by verifying the contents of 550.PBP, and if everything checks out, launch a 5.50 Gen-A installer that looks suspiciously like Sony's 5.50 official installer.
Since this is a full FW flash, it'll take a few minutes to complete. Once you're done, you can delete the UPDATE folder (and its contents, including 550.PBP), and just drag the 5.50 Gen-B update folder to \PSP\GAME and run that in much the same way as before. It'll take 1-2 seconds to patch the CFW. The 5.50 Gen-B2 patcher works identically.