Here's an example Apex trigger that updates the Account Number field on an Account record with "Test" followed by a random number:

```java

trigger UpdateAccountNumber on Account (before insert, before update) {
    for (Account acc : Trigger.new) {
        if (acc.AccountNumber != null) {
            acc.AccountNumber = 'Test' + Math.round(Math.random()*1000);
        }
    }
}
```

This trigger is set to run before insert and before update, which means that it will update the Account Number field before the record is saved to the database.

The trigger checks if the Account Number field is not null before updating it. If the field is not null, the trigger generates a random number between 0 and 1000 using the Math.random() method and adds it to the string "Test". The new Account Number value is then set on the Account record.

Note that this trigger will update the Account Number field for all new and updated Account records, so you should test it thoroughly before deploying it to your production environment.
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