sudo apt install capnproto
This installs v0.7.0 in my ubuntu 20.04. However, i require v0.8.0 to be installed.
Also, i need v0.6.1 to be installed additionally for backward compatibility.
Any solution for these two cases ?
You will probably need to install from source rather than use a distro package. Unfortunately, it sounds like the Ubuntu distro package hasn't been updated in a while.
Instructions for building and installing from source can be found here: https://capnproto.org/install.html
Old versions are available by changing the version number in the download URL to whichever version you need.
Note that all versions of Cap'n Proto are backwards-compatible, so there should be no need to install older versions, unless you need to run a specific complied binary that was linked against a specific old version.