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