Adding Captcha to Existing Contact Form

Hello,

I’m new to WP and need to know how I add a captcha to an existing contact us form. The form already in the site but I don’t where I can access any settings to my form and how to add captcha to it. Can this be found in the settings for the form, if so then where I find this? If it should be through a plug in (I’ve tried few plugins but all have to create a new form”.

Thanks in advance for help.