- You go to Setup - users- select the user you want to deactivate or change its license.
- open the user details and freeze it.
- If the user was assigned just a salesforce license, you click edit on the details page and change the user license and profile to a your choice of selection.
- This will help you change its license.
- to help deactivate the user, you uncheck the "active" checkbox under the profile, then save. this help deactivate the user.
NOTE: If the user is an admin, you have to remove every form of reference attached to the user in any flow e.g case escalation, lead assignment rules etc.