Flip model

Is it possible to flip a model in blender? (Flip as is used in Microsoft office, Adobe everything, and probably almost everything else.)
I only ask because I’ve been trying to use a mirror modifier with an object, and it says ‘modifiers cannot be applied to multi-user data’. Can’t seem to stop this error, so I have to do it ‘manually’. If anyone knows how to sort either of these problems, please reply!
Solved. Messing around, pressed “u”, problem solved. An inauspicious first post!