You can request a manual from the company.
From your question, I assume you do not have the owner's manual. The link at the bottom of this page will take you to the Henry website, and you can download the manual.
You can download a manual here - http://www.henryrepeating.com/freestuff.cfm
Best to ask a gunsmith for assistance.
Do not atttempt this. Find a gunsmith.
Ask a gunsmith for help
The rifle does not need to be disassembled to clean it. However, if you want to, the owner's manual has instructions for PARTIAL disassembly. The link below will take you to the owner's manual for that rifle. Would suggest you print and keep a copy.
WHICH Henry? The antique Henry rifle made in the 1860's or the current Henry Repeating Arms Co rifle?
http://www.lymanproducts.com/lymanproducts/LymanUsersGuide.pdf
Recommend you let a gunsmith do it.
Best left to a gunsmith.
Recommend a gunsmith be allowed to do it.
This is best left to a factory trained gunsmith.