You should add e.g. "(c) 2021 Joe R. User" to the file headers (where you find such comments) to mark that you've made some changes, and claim your rights to them formally.
Check the licenses. They don't ask to mark any changes you do, so you don't have to do so.
But think of your gentle reader, a few years down the road and half a world distant (or even yourself, in a few weeks ;-): You want them to understand what is going on, perhaps do some changes in a different way or elide them altogether. Today presumably you keep the code under some version control system like
hg. Make small, self-contained changes (not "weekly dump of changes"), write clear commit messages (not "Updated", "New version"), perhaps add in a
Changelog file with an overview of changes. Import any upstream packages marking their origin, version and so on clearly. If you update the base package, mark any relevant import commits clearly as such.
Think of any would-be user: Make sure to chase down any licenses that apply, record your findings and keep copies of the licenses at hand. This is also important for your protection: it isn't fun to get sued for inadvertently violating some license or software patent.