You can remove using:
yum remove php*
You will need to install the remi repo for Centos 6:
yum install -y https://rpms.remirepo.net/enterprise/remi-release-6.rpm
And later install php7 as follow:
dnf install -y --skip-broken --allowerasing --nogpgcheck \
php71-php-bcmath \
php71-php-cli \
php71-php-common \
php71-php-fpm \
php71-php-gd \
php71-php-imap \
php71-php-intl \
php71-php-ldap \
php71-php-mbstring \
php71-php-mcrypt \
php71-php-mysqlnd \
php71-php-pdo \
php71-php-pear \
php71-php-json \
php71-php-pecl-apcu \
php71-php-pecl-geoip \
php71-php-pecl-igbinary \
php71-php-pecl-imagick \
php71-php-pecl-jsond \
php71-php-pecl-memcache \
php71-php-pecl-redis \
php71-php-pecl-xdebug \
php71-php-pecl-zip \
php71-php-pgsql \
php71-php-phpiredis \
php71-php-process \
php71-php-pspell \
php71-php-soap \
php71-php-xml \
php71-php-xmlrpc \
php71-runtime
ln -s /etc/opt/remi/php71/php.d /etc/php71.d
ln -s /etc/opt/remi/php71/php.ini /etc/php71.ini
ln -s /etc/opt/remi/php71/php-fpm.conf /etc/php71-fpm.conf
ln -s /etc/opt/remi/php71/php-fpm.d /etc/php71-fpm.d
Hope this help you.