How do I assign shift key to constrain resizing object proportions?

For some reason, Photoshop was removed from my installed applications and I've had to re-install and now have lost all of my customized settings and preferences... It will take time to find and re-assign but will need help also. A big thing is that the shift key no longer holds proportions when re-sizing an object. I've scoured the menus and this is why it will be a daunting task to re-discover the years of work I've put into customizing Photoshop. Is the 'Legacy' choice in preferences maybe what I want. I don't want to upset the apple cart and make things even worse by making wrong choices. I'll start with the simple most annoying and work my way through this... How do I assign the shift key to hold and constrain proportions while I am re-sizing an object on the screen? TIA for. help