Article 138 allows taxpayers to correct errors or omissions in previously filed returns. If a mistake is discovered, the taxpayer must file an amended return electronically within thirty days of the discovery. This amendment must be made before the expiry of the assessment limitation period specified in Article 157. An amended return filed within this window is legally treated as the original return for the purposes of applying the Tax Law. This provision encourages voluntary disclosure and accuracy, allowing businesses to rectify honest mistakes without immediate adverse legal consequences.