PageNotFoundError.aspx is a SharePoint default Error page which is resides on <SharePointrl>/pages library.
As you can see it is by default using Layout called Error.
If you want to edit the exiting page you can simply check out and Edit the page in the browser.
If you can want to change the Layout you can change the Page Layout out by editing the page layout.
If you are good at PowerShell scripts, you can Update the custom page Url in Site Collection property.
$spsite = Get-SPSite “http://<SharePoint>”
$spsite.FileNotFoundUrl = “/pages/myerror.aspx“