In new API you should use

self.write({'state': 'approved'})

or

self.state='approved'

If you write many values better way (only one trigger a write action) is:

self.write({’x’:1,’y’:2,’z’:4})

See:
https://media.readthedocs.org/pdf/odoo-new-api-guide-line/latest/odoo-new-api-guide-line.pdf
https://github.com/nbessi/odoo_new_api_guideline
https://www.odoo.com/documentation/8.0/reference/orm.html