online application, missed few details, how to add?

[John1000s]

For my kids, I've filled two online application Tier 2 general child. On one child's application, I've missed to fill a section which asks for the other child's information. (Its about the other child, travelling.)

I've submitted the online application already. How can I amend it now? Can I write a letter and add it while submitting the documents?

[Lucapooka]

If this is a visa application (rather than leave to remain) you simply write any changes on the printed version of the form that you will be submitting to the VAC in that location.