I have a *.deb package. I install it. It creates a directory structure in /opt/welcome/net/. I manually delete the /opt/welcome/net directory.
When I reinstall it gives me error which is like:
Downgrade from 3.x to 2.0.0 is not allowed
dpkg-warning old preremoval script returned error exit status 5
Unsupported downgrade or upgrade
new preremoval script returned error exit status 1
Errors were encountered while processing *.deb
Plz give me some suggestion how to reduce this error.

OK, I'll need a few details.

What distro are you running?
What are you trying to install?
Are you sure the Dependencies have been fully satisfied?
Are you installing via a package manager or command-line?

This is my own debian package.
I have this in my postinstall script:
# Do nothing for abort-remove
if [ "$1" == "abort-remove" ]
then
exit 0
fi

# Do nothing for abort-upgrade
if [ "$1" == "abort-upgrade" ]
then
exit 0
fi

I have this in my postrm script:
NEW_PKG_MAJR_VER="$(echo $2 | cut -d"." -f1)"
echo "NEW_PKG_MAJR_VER = $NEW_PKG_MAJR_VER" >> ${LOG_FILE}
if [ $NEW_PKG_MAJR_VER -lt 3 ]
then
errrmsg 5 "Downgrade from 3.x to $2 is not allowed"
fi
fi

if [ "$1" = "failed-upgrade" ]
then
errrmsg 1 "Unsupported Downgrade/Upgrade"
exit 2
fi
I know it is because of postrm.
I want to know how can I avoid this error.

Oh, I see.

In that case I won't be able to help as my knowledge of creating .deb packages is non-existent.

Sorry.