Lack of details is confusing us. I guess ure on cex ps3, cfw 4.55 (zero or ferrox), and u want to downgrade cfw version. U need two steps, first u need to install a 9.99 downgrader firmware to go back to 3.55. U can choose rebug, rogero, habib... any of them do the work. Install the downgrader from recovery, then also install the target cfw from recovery.
If ure not on cfw 4.55 but using a 4.55 spoofer u need to remove spoof first, usually by first installing from recovery the same cfw version u have installed (making a total of 3 recovery cfw installs in a row).
If ure on 4.55 original firmware then nothing can help u downgrade but a hardware flasher soldered to mainboard.
I guess by 4.2.1 u mean 4.21, but there is no reason to go so low and old. Rebug 4.46.1 is the latest release.