There are lot of brute force attacks over default service ports and hence it is highly recommended to change the default port as a first steps security measure. The default SSH port of Linux distros is 22. You can change this to something else by following below steps:

1) Login to the server via SSH as root:

ssh root@hostname/IP

where hostname/IP should be the host name / IP address to connect to the server.

2) We will need to edit sshd_config file and hence it is recommended that you backup original file before proceeding:

cp /etc/ssh/sshd_config /etc/ssh/sshd_config_backup

3) Open sshd_config in your favorite text editor (vim / nano):

vi /etc/ssh/sshd_config

-OR-

nano /etc/ssh/sshd_config

4) Navigate to line containing text #Port (which is currently commended out with a # sign). Remove # and replace port 22 with your preferred new port # (eg. 2220).

#Port 22 should be replaced with

Port 2220 (without #)

5) Save your changes and close the sshd_config file

6) Important – If you have firewall running on the server, you will need to edit the rules and allow newly entered port # in firewall exception list. For default iptables, you can use below rule (for custom port 2220):
iptables -A INPUT -p tcp -m tcp –dport 2220 -j ACCEPT

7) Restart sshd service by running  /etc/init.d/sshd restart