File to edit limits is limits.conf which is located at : 
For Ubuntu distribution : /etc/systemd/system/mysql.service.d/limits.conf.
For CentOS distribution : /etc/systemd/system/mysqld.service.d/limits.conf.
If the files is not already present then create it.

Step 1 : Edit the file limits.conf
You can manually open the file or you can also edit file with the following command:
systemctl edit mysqld  # RPM platforms
systemctl edit mysql    # Debian platforms

For example :
[Service]
LimitNOFILE=50000

Step 2 : Tell systemd to re-read the config files to apply the changes with the following command:
systemctl daemon-reload

Step 3 : Restart MySQL with the following command:

systemctl restart mysqld.service   # RPM platforms
systemctl restart mysql.service       # Debian platforms