The Centers for Medicare and Medicaid have posted a guide for converting back and forth between ICD-9 and ICD-10. You can find the guide You can find the guide by clicking here. Look for the zip file labelled "2015 General Equivalence Mappings (GEMs) – Diagnosis Codes and Guide."
This zip file contains full instructions for doing a mapping from ICD-9 to ICD-10 in the file labelled Dxgem_guide_2015.pdf.
If this 21 page document seems intimidating, you should know that you only need to read the first 18 pages. If that's still too intimidating, be sure to read our next blog post.