chmod is the tool to change the permission on UNIX based systems.
For the options consult wikipedia.
Why is it worth to mention it here? If you change the permissions recusively ofer a directory tree, usually you do not have the right permissions for directories and for regular files.
You can avoid that by using find, as shown in the example:
find . -type d -print | xargs chmod 755
find . -type f -print | xargs chmod 644
Type “d” applies the change to directories, whereas type “f” applies it to files. find “.” means to all files starting in the current directory.