Mixed Content Warning only when I hit the calender button

Mixed Content Warning only when I hit the calender button

Hello There,

I get a mixed content warning the moment I hit the calender in my request form.

jquery.js:3 Mixed Content: The page at 'https://www.ossiachersee.net/urlaubs-anfrage/' was loaded over HTTPS, but requested an insecure image 'http://code.jquery.com/ui/1.11.4/themes/smoothness/images/ui-icons_222222_256x240.png'. This content should also be served over HTTPS.

This is a wordpress site. How do I get this fixed? I am an absolute beginner.....

Thank you for your help!

Kind Regards, Inge