According to the following article you have 2 options to unsubscribe:

https://doc.sitecore.com/developers/exm/90/email-experience-manager/en/the-unsubscribe-options.html

First option is to put the /sitecore modules/Web/EXM/UnsubscribeFromAll.aspx or /sitecore modules/Web/EXM/Unsubscribe.aspx to your email footer. With this link users can unsubscribe from the newsletter directly from their mailing. Sitecore will change the url for RedirectUrlPage.aspx.

Second option is to create a Sitecore Form that uses the Email Experience Manager Client API or create a custom form with an API controller which unsubscribes the user with the Unsubscribe() api call.

According to the following article you have 2 options to unsubscribe:

1. First option is to put the /sitecore modules/Web/EXM/UnsubscribeFromAll.aspx or /sitecore modules/Web/EXM/Unsubscribe.aspx to your email footer. With this link users can unsubscribe from the newsletter directly from their mailing. Sitecore will change the url for RedirectUrlPage.aspx.

2. Second option is to create a Sitecore Form that uses the Email Experience Manager Client API or create a custom form with an API controller which unsubscribes the user with the Unsubscribe() api call.