How can I link the mesh data of several objects so they all use the same? I have alot (like 50) duplicated meshes. i used SHIFT-D to duplicate the objects ,but then read that ALT-D saves memory. is there a way to link the mesh data (like using ALT-D) after i have used the wrong duplicate? attempting to go back and use ALT-D would take a long time.

The meshes i duplicated are all the same shape and size and only have different rotations.
Select the objects you want to transform and then shift select the object that you want to clone to make it the Active Object.

Press the space bar and type "link data".
(or press Ctrl+L and select "Object Data", or use the menu Objects>Make Links Object Data)

The selected objects will then use the data from the active one.