How to write files in hosting in admin dashboard?

You basically have two options:

1. Use any of the available File Manager plugins for WordPress. Just Google them, literally that phrase I just used.
2. Implement a custom PHP code in your theme or custom plugin which will utilize any of the PHP functions for manipulating the files. For example, check these:
   • file_get_contents() – For reading files
   • file_put_contents() – For writing files. This will also create a new file if it doesn’t exist.
   • unlink() – For deleting files
   • chmod() – For changing permissions
   • chown() – For changing owner

There are also a ton of functions for more advanced file manipulation in PHP. Check this for reference: Filesystem Functions