Short answer - you cannot - for your own good.

This is a fundamental of double entry bookkeeping, every movement has a credit and a debit. It makes it easier to track where your money is going.

It is poor accounting practices to just re-write entries in the ledger. If you made a mistake, make a correcting entry that cancels out the old stock move. Like if you shipped it to a partner in the computer by mistake, create a return with a note that this was just a computer error.

If your database is getting too big, I would see about exporting the information somehow.

OpenERP prevents you from deleting stock moves that are not in the DRAFT state. If a stock move is DONE by mistake (should not have been completed), then you need to create another stock move that is the opposite of the mistake move.

update stock_move set state = 'draft' where id = ?;

Then you can delete them from within OpenERP safely. Or if safety is not a concern...

delete from stock_move where id = ?;

I don't recommend the latter but I've had to do it when a CSV upload test generated several hundred test records and I needed them gone for the next iteration of testing.

In Odoo 11 onwards, When Delivery or shipment is in the done state, There is Unlock button.

When you click on this, Order become unlock and you will get an option to edit a Done Qty in Move lines.

If you want to just cancel the order, you can simply provide done qty as 0 and again lock it.

Effect of this is, you onhand qty for a product will decrease for shipment or increase for delivery as stock move receive 0 qty. It is the most safest way as it takes care of all stock related and inventory related odoo flow.