Hi Sarat and welcome to the RH community.
First of all, make sure that you do not have access to the
source files that produced the CHM. If you have access to the
source it will save you lots of time and trouble. Ask around. Maybe
someone knows something about where the source is. Note that
depending on what package was used to create the CHM and how old
the source is, you may not find an .XPJ file.
If you really can't find any source, you can decompile the
CHM to a folder (a similar proccess to extracting a zip file) using
RH's inbuilt HTML Help Studio tool. This recreates the topics in a
series of HTM files. Create a new RH project and import them in.
The decompile also generates any browse sequence, glossary, TOC and
index files. These you can copy into your new project's directory
to overwrite the ones already there. Take backups before you do
this just in case.
This will give you the basis of your help file but you may
find that if topics had conditional build tags, variables or
snippets that these will not be present. You can always copy across
the , rhbuildtag.apj, rhsnippet.apj and rhvariable.apj files from
another project if they use the same ones to start the process of
reinstating them.
This
article covers what I have added in some more detail.