First you have to decide where you want to give onchange function and then give attribute on_change to that field,, for Eg:- let our field is sample_field

<field name="sample_field" on_change="onchange_sample_field(arg1, arg2)"/>

Here onchange_sample_field is my function name

Next step is to define that function:-

Define what you have to do in that function,, function definition Eg:-

    def onchange_sample_field(self, cr, uid, ids, arg1, arg2, context=None):
        function body
        return result

my py file

class instalment_receipt_entry(osv.osv):
    _name = 'instalment.receipt.entry'
    _columns = {      'agreement_number_id': fields.many2one('agreement.number','Agreement Number'),

                               'receipt_no':fields.char('Receipt Number',size=64),

}

and my xml file 

   <field name="agreement_number_id"/>
    <field name="receipt_no"/>

how to onchange this