How to Display a custom file while in Maintenance Mode

[FaeriesDance]

So 2 years ago I looked this up and had figured this out.  Every time the site was put into maintenance mode it automatically displayed an html page that I created and had sitting in the top level directory.  I believe I had made changes to the Bootstrap file. 

I recently upgrade my magento version, and whatever I did before got overwritten.  I've been searching for an hour and can't find the instructions again, and I don't even remember where the Bootstrap file is. Can anyone help?

Thank you,

Adrienne

PS - I prefer not to do this as an extension.  Thanks.

[Rakesh Jesadiya · Accepted solution]

You can find maintenance page for magento 2 from below path,

pub/errors/default/503.phtml

You can modify above file for maintenance changes.