How I can rescale my assembly which is downloaded for grab cad in fusion 360 ??

It seems like you're facing a common issue when working with assemblies from different sources with varying scales. To address this, you can use Fusion 360's built-in tools to rescale the entire assembly. Follow these steps:

1. **Open the Assembly:**
- Open your assembly file in Fusion 360.

2. **Create a New Design:**
- If your assembly is not already in a new design, create a new design by right-clicking on the assembly in the Browser and selecting "Save As New Design."

3. **Check Units:**
- Go to the "Modify" menu and select "Change Units" to ensure that the current units are set correctly.

4. **Use the Scale Tool:**
- Go to the "Modify" menu and select "Scale."
- Choose the entire assembly as the selection set.
- Specify the scaling factor. For example, if you want to reduce the size to half, enter 0.5. If you want to double the size, enter 2.

5. **Adjust Constraints (if necessary):**
- After scaling, you may need to adjust constraints and relationships between components if they are not behaving as expected.

6. **Save Changes:**
- Save your changes.

This should help you scale the entire assembly uniformly. Keep in mind that if the original assembly has constraints, relationships, or custom parameters, scaling might impact them, so be prepared to review and possibly update those.

If you find that the scaling operation is not sufficient or causes issues, consider contacting the original designer or the GrabCAD community for advice. In some cases, you might need to scale individual components separately, especially if they have complex relationships with each other. Remember to save your work and make backups before making significant changes.

The problem might be caused by using the wrong units. Try changing the units to millimeters, not inches.