If it has not yet been a year since that decision was issued, you are still within the time frame to appeal the decision, and you should file a Notice of Disagreement. This must be submitted in writing and specify exactly your disagreement with the decision you received. You should refer to your decision notification letter and the attached VA form 4107, Your Rights To Appeal Our Decision, for further guidance on filing a Notice of Disagreement. You may download a copy of VA Form 4107 at: http://www.vba.va.gov/pubs/forms/4107.PDF
If you do not have a copy of your decision notification letter, VA can send that information to you. You may contact the VA regional office to make that request by going to VA's web messaging site, https://iris.custhelp.com
If it has been more than a year since VA made the decision that you believe is incorrect, that decision is now final. If you wish to have that decision reviewed, you will need to specifically identify the error you believe was made, such as the determination of service connection, the effective date, or evaluation for a claimed disability. You will also need to identify the factor(s) that resulted in an incorrect outcome, such as the incorrect application of the law.
It may be best to discuss your personal situation with a VA or Veterans Service Organization representative who can advise and assist you with your claim. VA offers a web site list of most National, State and local veterans service organizations at: http://www1.va.gov/vso/