How can I upload a model from Fusion 360 to Grab CAD ?

On the First page "Library" Top right " UPLOAD" in Red

You'll need to export your 3D model first from Fusion 360. Open your model, click 'File' dropdown on the top left and click 'Export' and choose your desired 3d file output.

To upload your file to Grabcad, hover over your profile picture on the top left and click 'My Models'. Click on 'Upload a New Model' then choose your file.

I export the Fusion model first (File>Export) usually *.step or *.iges work well Fusion is very good at exporting because anything you don't want to include can just be switched off in the browser tree (click the eye) some other software I've used exports everything.