In your cpanel, find the FTP accounts icon and click, scroll down to the Account management area of the page and find the account in question. To the far left of that users name are several icons, click the one titled Configure FTP client.
On the bottom of the new page you should see the port number listed there for SFTP access. You will of course need to setup your FTP client to connect using SFTP as well.
You may wish to consider TLS rather than sFTP. TLS is available by default with cPanel in WHM > FTP Server Configuration at the top under "TLS Encryption Support" where it has "Optional" set.
TLS is actually more secure than sFTP as you do not need to provide a jailed shell to users in order to enable TLS. TLS would also be supported on all FTP user accounts even those you'd created in cPanel's FTP Accounts area.
Almost all modern FTP clients such as FileZilla have support for TLS. Please note that the type of TLS supported is FTPES - FTP over explicit TLS/SSL not FTPS - FTP over implicit TLS/SSL when configuring your FTP client.