Please tell me how can i convert files from one format to anther using grabcard platform

Yes and No…
Here on GrabCAD, you can convert any previously-uploaded files but into universal extensions only.
Check this article:
https://help.grabcad.com/article/143-converting-a-file-to-another-format

To convert your files into extensions other than those mentioned in the article, use an online converter instead.

In your case, those files could be imported directly into Fusion 360.
Check this question:
https://forums.autodesk.com/t5/fusion-360-design-validate/fusion-360-free-version-importing-files-from-other-softwares/td-p/6580368

There is another way, you import the files on workbench project once importing has been complete, select download as to choose format. Then once conversion is complete file will be downloaded