If you want to unsubscribe an email on a one-off basis, you can do it in the lead record.
Go to the lead database, search for the lead you want to unsubscribe, click the ID number, then scroll down to the "Unsubscribed" data value on the "Lead Info" tab, and check the box.
If you are unsubscribing a bunch of leads from some lists, a smart campaign would be much faster.
Smart List - your lists/filters that qualify your leads
Flow - Change Data Value - Atrribute: Unsubscribed - New Value: True