How to show the PayPal checkbout button in my form?

3. Also would be lovely to know how can I change my submit button to 'paypal checkout button'. I changed it in settings but still...

Go to the edit mode of your form, select your payment field, click on it's gear icon, and on "Edit Properties", then enable the Checkout button:

Let us know if you need more help.

yes. I did it as well but still doesn't change it.

I believe you are referring to this form: http://www.jotformeu.com/form/61014414245342 

You are correct, it is not taking effect, however you can add the image of the button, example:  

1) Select your submit button.

2) Click on the "Button Image" option and paste the URL of the image: https://www.paypal.com/en_US/i/btn/btn_dg_pay_w_paypal.gif 

Let us know if you need more help.