Every email (i.e. newsletters), except “transactional emails” should have a visible unsubscribe paragraph, like a link at the bottom of the message. Scroll down to the bottom and look for “Unsubscribe”, usually a small text but it should always be there. You can also press Ctrl + F (command + F on a Mac) to open the search field in your browser or client, then type the word unsubscribe to search for it. Just click the link to unsubscribe!
The GDPR (EU), CAN-SPAM Act (US), and similar laws, require that all emails must contain a visible “unsubscribe mechanism”, which can be a link but can also be an email address to be contacted to send your request. Also, the email must contain physical mailing address associated with the sender. In the GDPR the request to opt-out must be honored without “undue delay”, in the CAN-SPAM Act your request to opt out must be honored within 10 business days.
Services like Gmail also give you the option to unsubscribe from a link next to the sender’s email address.
If an email fails to include a way for you to opt out you can actually report them to the relevant authorities in your country. Also, if you can’t get unsubscribed, be advised that most used email services like Gmail or Outlook have a [Spam Button] that will block all messages from that sender. Important: Spam button should be used ONLY for real spam. As a general rule just unsubscribe from the legitimate commercial emails you receive with their included Unsubscribe links.