I had download the form source file and uploaded it to my server.

I had download the form source file and uploaded it to my server. Where can i edit the thank you page redirect url? Is it still using this platform?

I have inspected one of your forms and I saw that you have set redirect URL inside of thank you page settings. 

Please note that when using the Source Codes, every time you change your form in the form builder, you must update the form codes on your site by re-embedding the form. This is because the source codes that are currently embedded to your page are not automatically updated. 

To resolve your issue re-embed your form:

How to get the Full Source Code of your Form

If you do not want to re-embed your form each time form is edited, that I would suggest you to embed your form with iFrame code:

Getting the form iFrame code

Let us know if you need further assistance.