1. backup all databases first
2. service mariadb stop
3. yum remove mariadb mariadb-server
4. yum install vim epel-release -y
5. vim /etc/yum.repos.d/mariadb.repo
6.then paste this lines and save it :
# MariaDB 10.2 CentOS repository list - created 2018-02-15 08:21 UTC
# http://downloads.mariadb.org/mariadb/repositories/
[mariadb]
name = MariaDB
baseurl = http://yum.mariadb.org/10.2/centos7-amd64 gpgkey=https://yum.mariadb.org/RPM-GPG-KEY-MariaDB
gpgcheck=1
7. yum install MariaDB-server MariaDB-client -y
8. systemctl enable mariadb
9. systemctl start mariadb
10. mysql_upgrade