@SaiKiran
please have a look at this page:
https://senbox.atlassian.net/wiki/spaces/SNAP/pages/30539785/Update+SNAP+from+the+command+line
At the bottom is described how you can configure the proxy without using the SNAP GUI.
If you can use the GUI you can go to Tools / Options in the menu. Switch to the WWW tab and configure the proxy.