The regular way of getting access to the files now, is to allow VirtualBox to automount the shared folder (which will make it show up under /media/sf_directory_name) and then to add your regular Ubuntu user to the vboxsf group (as root #).

# usermod -aG vboxsf <youruser>

By default, without manual action, the mounts look like this,

drwxrwx--- 1 root vboxsf 40960 Oct 23 10:42 sf_<name>

so the vboxsf group has full access. By adding your user to that group, you gain full access. So you wouldn't worry about changing their permissions (which don't make sense on the Windows host), you just give yourself access.

In this specific case, this is the automounted Shared Folder,

Ubuntu               214153212  31893804 182259408  15% /media/sf_Ubuntu

and it is that directory that should be used to access to the Shared Folder, by putting the local user into the vboxsf group. If you want a 'better' link under your user's home directory, you could always create a symbolic link.

ln -s /media/sf_Ubuntu /home/m/Desktop/vbox_shared

You will need to reboot your VM for these changes to take effect

If you manually mount the shared folder, then you need to use the relevant options on the mount command to set the folder with the right ownership (i.e. the gid, uid and umask options to mount). This is because the Host OS doesn't support the same permission system as Linux, so VirtualBox has no way of knowing who should own the files.

However, I strongly recommend just configuring the shared folder to be auto-mounted (it's a setting on the Shared Folder configuration in VirtualBox itself).

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For the avoidance of doubt, I do not believe you can change permissions normally anyway, on that filesystem if it's mounted in the regular way,

tony@jabba:/media/sf_name$ ls -l tst.txt
-rwxrwx--- 1 root vboxsf 2283 Apr  4  2012 tst.txt
tony@jabba:/media/sf_name$ sudo chown tony tst.txt
[sudo] password for tony: 
tony@jabba:/media/sf_name$ ls -l tst.txt
-rwxrwx--- 1 root vboxsf 2283 Apr  4  2012 tst.txt
tony@jabba:/media/sf_name$