The Collapse command is available after an assembly has been exploded with the Explode or Automatic Explode commands.
Choose Home tab→Select group→Select Tool.
Select the part you want to collapse.
Choose Home tab→Explode group→Collapse 
.
Tip:
You can collapse several parts in one operation by holding the Shift key and selecting the parts you want to collapse.
You can collapse all the parts in a bound subassembly by selecting one of the parts.