Make sure system is uptodate.
yum -y update
Chnage the directory to tmp and run the follwing commands.
cd /tmp/ curl -sS https://getcomposer.org/installer | php mv composer.phar /usr/local/bin/composer chmod +x /usr/local/bin/composer
Now you can check the composer version as follows
[root@localhost bin]# composer -V Composer version 1.6.5 2018-05-04 11:44:59