What legal form to file if can not be a witness due to not remembering

There is no "form" to file and you still can and must be a witness if subpoenaed (issued a legal summons or paper to appear and testify). You cannot escape testifying by claiming lack of memory: all you can do is testify honestly to the best of your knowledge. It is permitted to say "I don't remember" or "I am not sure." You can refuse to answer specific questions if the answers could incriminate you personally criminally--i.e. support criminal charges against you. However, you still need to appear and in response to each question that could incriminate you, say "I refuse to answer on the basis of the 5th Amendment" or something similar, invoking that right.
If an officer wrote down some of your answers or information incorrectly before, if you did not sign the report (thereby essentially swearing to its accuracy), you may have to say something like "the officer didn't understand what I said" or "the officer did not record my answer correctly." If you did not sign the report, it's how another person recorded what you said, not what you certified to. The court may choose to believe the earlier records vs. what you say now, but there is nothing you can do about that.
If you did sign the report (or otherwise previously give information or testimony under oath), it becomes more sensitive--you could be accused of perjury, since you did essentially affirm that the information was correct. If this occured, retain an attorney and go over the potential discrepancies with him/her, and let he/she advise you how to answer.