I have just upgraded to the new 5.7 but it is slow and keeps crashing. Is there anyway to go back to the old version without losing the program. Not sure I still have my serial key for this either. Please can someone assist.
Sure, download Lightroom 5.6 and install.
I'm a little worried that you state "Not sure I still have my serial key for this either". Not only do you need to find the serial number for Lightroom 5, but you need to develop a method of keeping your serial numbers (for all software) is a safe place where you know you can find them when needed ... no excuses!