Edit your Nginx virtual host config file and the redirection could be done as follow.

non-www to www

server {
    listen 80;
    server_name example.com;
    return 301 $scheme://www.example.com$request_uri;
}

server {
    listen 80;
    server_name www.example.com;
    ## here goes the rest of your conf...
}

Similarly, you can redirect www to non-www by

server {
    listen 80;
    server_name www.example.com;
    return 301 $scheme://example.com$request_uri;
}

server {
    listen 80;
    server_name example.com;
    ## here goes the rest of your conf...
}

Done =)

Reference: StackOverflow – Nginx no-www to www and www to no-www