Same as on Unix, use chmod. Something like (untested)
chmod -R 0775 dir/
The -R will recurse through the subdirectory, remove if not needed.
Right Dave, it worked with this command, thanks.
Ivan, I needed to change permissions in two folders due this answers at a OS2 command line:
invalid permissions on directory C:\MPTN\ETC/samba/private/msg.lock': has 0777 should be 0755
invalid permissions on directory C:\MPTN\ETC/samba/private/msg.sock': has 0777 should be 0700
still fighting with Samba implementation, installing from rpm package on OS2 Warp 4.52 is not enough, then I have to move/add myself some files and folders in C:\MPTN\ETC to get it work