What is can I do if I want to recant my court testimony but am afraid of being charged with perjury?

Question Details:

I am the key witness in a murder trial. I was threatened by officers to say that I saw the defendant commit the crime. He was charged with second degree murder and sentenced to 47 years. Today I am willing to go forward and recant my statement but fear being charged with perjury? In court can I just answer questions based on my new affidavit and plead the fifth to any other questions?  

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