You can do two things.
1) Call your Congressman and Senators and tell them to sign H. Res 556 and S. Res 281, introduced by Rep. Walter Jones and Sen. Charles Grassley, calling on President Trump to release all the secret JFK files and to reject any requests from the CIA and the FBI for continuing secrecy. These petitions, also signed by leading liberal Democrats such at Sen Pat Leahy and Rep. John Conyers, show that full JFK disclosure enjoys wide support.
2) Sign the online petition to President Trump created by Coalition Against Political Assassinations.
The JFK Act of 1992 Requires the government to release all of the records on the assassination of President Kennedy by October 26, 2017. Only the President can continue to withhold records and there is no legitimate reason to do so except to protect the guilty.
Source: Petition · Donald Trump: Free the JFK Files · Change.org