Sure, here is the Apex trigger code that changes every account number to "test" plus a random number:

```
trigger changeAccountNumber on Account (before insert, before update) {
  for (Account record : Trigger.new) {
    record.AccountNumber = 'test' + Math.random();
  }
}
```

This trigger will run before any insert or update operations on the Account object. For each Account record that is affected by the operation, the trigger will change the AccountNumber field to "test" plus a random number. The random number will be generated using the Math.random() function.

I hope this helps! Let me know if you have any other questions.
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