No it is not true. You have up to three years from the filing deadline to submit an amended return.
If you need to enter additional information, delete something, or change something on the the tax return you already filed, you have to wait until your return has been fully processed, then you can prepare an amended return, called a 1040X. You cannot e-file a 1040X. It must be printed, signed and mailed to the IRS.
After you print, sign and mail your amended return—1040X— to the IRS wait about 3 weeks and then you can begin checking on it at the IRS website. Be aware that it can take at least 3-4 months for an amended return to be fully processed (or sometimes even longer.)