My feeds are stored in /var/lib/phptimeseries e.g as feed_137.MYD. How can I manually edit them?
You can’t really. What do you need to do?
I want to remove old data. I failed to find related table in phpmyadmin.
If you look in the ‘Learn’ section, there you will find details of the format and structure of your data. My approach would be to write a “data swabber” program that will read the file and write to a new file the parts that you want, maybe allowing you to edit the data with a GUI, and of course observing the same structure and including the new metadata. Then you could delete or rename the old file and give the new one(s) the same name as the old.
I’ve no idea whether that would work, but I can’t see a reason why it should not - always assuming everything is done correctly.
I’ve thought about writing that software - but what you’ve just read is as far as I’ve got. 
And with the amount of OEM stuff I’ve got on my plate at the moment, it won’t happen any time in the foreseeable future.