Sorry I am new to SolidWorks but I wonder if I change the dimension in assembly, will it change the dimension in the parts automatically?
Yes, you can edit part in assembly mode.
It will change the part when it is opened, if your using a PDM system, you will need to check the part and assy out.
"automatically" would be the key word here. I think if you edit a sketch dimension of a part in the assy it should update after you've made the change unless it's tied to another part in context and then if you do a rebuild it should update then.
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