You can change a user's password in their Change Password tab.
To change it, enter the new password to the two fields and click the 'Save' button below them.
Deleting Users
You can delete a user by clicking the trash can icon on the right side of the username.

Notice that only users on the Account Admin user level can delete other users.
After clicking the delete button a modal will appear, asking if the assets created by the user about to be deleted should be transferred to a different user. To do so, select the user from the dropdown in the modal. If a user is not selected, the assets will also be deleted.
Click 'Delete User' to confirm.