You can reposition parts only after you have exploded an assembly using the Explode or Automatic Explode commands.
Choose Home tab→Explode group→Reposition.
Select the part you want to reposition.
Select the part to be used as a reference part.
Note:
An arrow is displayed that points toward where the part will be repositioned. If the arrow does not point in the direction you want, select a different reference part.
Click to reposition the part.
Tip:
If the reference part you selected in step 3 is the last part along the explode vector, a dynamic arrow is displayed. Position the cursor so that the arrow points in the direction you want to reposition the part, then click.