First of all, you need to list your right.
ls -l /your_directory/
-rwxr-xr-x 1 57 Jul 3 10:13 file1
-rwxr-xr-x 1 57 Jul 3 10:13 file2
to translate the result in numerical permission use this : R = 4, W = 2, X = 1.
So on this 2 file the permission are 755.
After getting your right, you must use the chmod command to change the right the way you want :
chmod 775 /your_directory/
This command will only change the right on the directory, but not on the file inside.
If you want to change the right inside too, then do this command :
chmod 775 /your_directory/ -fR
The -fR will force the recursivity. (use it only if you are sure of the right you want to apply, and the file inside the directory.)
If you only want to change the right on file1 then :
chmod 775 /your_directory/file1
And that the way the cookies crumble!
/!\ Be carefull, misuse of this command can destroy all your OS permissions and lead to malfunction of it. /!\
ps : look there to find out more information about chmod.
Hope this will help you.