How to Create business rule or Field trigger on Business object

Summary:

Need help to create field trigger there is 1 column in table which is primary key invoice_id and its varchar type when user doing update trigger will validate if that value is exists in system and count is 1 then I want to update the table record and if not then insert new record with same invoice id but I want to append or concat sequence with invoice id like

invoiceid+'_'+1

invoiceid+'_'+2 etc

how to achieve this with help of field level trigger or business rule