I would like to upgrade DXA java 1.5 to 1.6, Is it only JAR changes or Do I need to install in at CMS end as well ? What are the steps to be followed to upgrade the DXA Java framework from 1.5 to 1.6 ?
The DXA upgrade procedure is documented here: http://docs.sdl.com/LiveContent/content/en-US/SDL%20DXA-v7/GUID-7BBD251F-E5AB-4A40-B57A-51E3F7635C17
Admittedly, this is DXA 1.7 docs, but the procedure didn't really change in DXA 1.7; it just wasn't explicitly documented earlier.
Please take a look at the SDL docs for the new features Changes introduced in DXA 1.6. Looking at the core module seperation feature, there seems to be quite a bit of change in CMS side as well.
Its always advisable to upgrade both CMS and webapp together. DXA CMS Import script is smart enough to identify the existing DXA versions in CMS and upgrade it. Importing the DXA Publications into Content Manager