Thanks.
Earlier I was using 'Discover' or 'Muon'.
After running 'sudo apt update', I had to accept 2-3 times. Now it is showing 439 packages to update.
(Deleted middle content from the code below to reduce size.)
jagdeesh@vbox-jld-lap-nep6:~$ sudo apt update
[sudo] password for jagdeesh:
Get:1 http://security.debian.org stretch/updates InRelease [94.3 kB]
E: Repository 'http://security.debian.org stretch/updates InRelease' changed its 'Suite' value from 'stable' to 'oldstable'
N: This must be accepted explicitly before updates for this repository can be applied. See apt-secure(8) manpage for details.
Do you want to accept these changes and continue updating from this repository? [y/N] y[code]
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[code]Get:39 https://cdn-aws.deb.debian.org/debian buster/non-free i386 Contents (deb) [780 kB]
Fetched 134 MB in 13min 49s (161 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
439 packages can be upgraded. Run 'apt list --upgradable' to see them.
jagdeesh@vbox-jld-lap-nep6:~$ [code]