On this case if you want to overwrite it you have to follow the following steps:

1. add account to dependencies on your module (__openerp__.py)
2. Look for the original code, in this case: (/account/data/account_data.xml)
3. Copy the entire record that you want to change and paste it on your module
4. Add the module name of the module account on your module (change sequence_sale_journal -> account.sequence_sale_journal)
5. Modify what you need to modify
6. Install your module

Account module

 <record id="sequence_sale_journal"
 model="ir.sequence">
             <field name="name">Account Default Sales Journal</field>
             <field eval="3" name="padding"/>
             <field name="prefix">SAJ/%(year)s/</field>
         </record>

Your module

<record id="account.sequence_sale_journal" model="ir.sequence">
            <field name="name">Account Default Sales Journal</field>
            <field eval="6" name="padding"/>
            <field name="prefix">SAJ/%(year)s/</field>
        </record>

Just simply

Go to Setting->Technical->Sequence and Identifiers->Sequences

Find Sales Order