Apologies if my technical nomenclature is wrong as I am new to Solidworks!
I have created an assembly from several parts, however, I now want to change which part is the datum for the assembly, i.e. the ground (and shadows) are alligned with it and the file always opens with it in the same position.
This is because I have since added a frame to the first part which I would like to be the base or datum. (In attached image, I want the 90 degree arc piece flats to be the base.
Any help much appreciated!