About Regenerating Assemblies and Parts Modified in Assembly
When changes are made to an assembly, it must be regenerated. Parts that are modified in an assembly must also be regenerated. Use the 
Regenerate command to update modified assemblies and parts after you make modifications. In large assemblies, or to customize regeneration, you can use the 
Regeneration Manager to select individual parts to regenerate.
Regenerate command to update modified assemblies and parts after you make modifications. In large assemblies, or to customize regeneration, you can use the Regeneration Manager to select individual parts to regenerate.By default the regenerate command identifies and regenerates all parts that were modified including the top level assembly and subassemblies. To reduce regeneration time, you can select the components to regenerate in Model Tree or using the Search Tool.
A part intersected by an assembly feature and modified in a separate session must be regenerated in the assembly for the modifications to be visible in the assembly.
Before regenerating a part with external references, the system regenerates the external references and then uses the updated values for the part.
When a model cannot be regenerated for any reason, it is restored to its state before the regeneration process and not modified.