In existing partners first we have to go in .py file (Python file) After that we have to decide where we want Search through the existing partners based on the lead's partner fields and then give start to end Path to that field.Use below .py code in python file:

 elif active_model.partner_name:
                partner_ids = partner_obj.search(cr, uid, [('name', 'ilike', '%'+active_model.partner_name+'%')], context=context)
                if partner_ids:
                    partner_id = partner_ids[0]
            elif active_model.contact_name:
                partner_ids = partner_obj.search(cr, uid, [
                        ('name', 'ilike', '%'+active_model.contact_name+'%')], context=context)
                if partner_ids:
                    partner_id = partner_ids[0]

        return partner_id

    def default_get(self, cr, uid, fields, context=None):
        res = super(crm_partner_binding, self).default_get(cr, uid, fields, context=context)
        partner_id = self._find_matching_partner(cr, uid, context=context)

        if 'action' in fields:
            res['action'] = partner_id and 'exist' or 'create'
        if 'partner_id' in fields:
            res['partner_id'] = partner_id

        return res