How to use Lock objects ..?

Hi,

I am having an item table.I wanted to know what is the best practice for using lock objects for update the table.

Let consider, say for example billing item table VBRP, where the key fields is Billing Document No. and Billing Document Item.

I want to update amount of multiple Billing Documents after locking VBRP, the information is available in internal table with the same structure as VBRP let say the internal table is it_vbrp

Say for eg.

Bill Docs           Bill Doc Item         Amount

90000001          1                             100

90000001          2                              50

90000002          1                              20

90000002          2                              40

90000003          1                              30

90000004          2                              60

Kindly let me know whether we can use 'UPDATE' statement instead of 'modify' from internal table.

Please let me know the Best Practise to be Used. If anyone is having BC414,kindly share with me.

Regards,

Fredric