I'm working on, and adding to, a GitHub-hosted project that includes this LICENCE.md (apparently the MIT licence verbatim):

Copyright (c) 2012 [Acme Corp]

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.


I've made changes, improvements etc (say 10% of the code, in ballpark figures) and publish the code to my own GitHub fork.

What should I do with this copyright notice? I'd like to update it (eg, just to add the name of my own organisation), but it says not to. How are these things normally managed? Add a separate copyright file?

You've got some options, jump to the end for the summary.
So let's break this one down...


Copyright (c) 2012 [Acme Corp]  


This is the Copyright notice and it belongs to Acme Corp.  It was claimed in 2012, which is relevant because Copyright does eventually expire.

If the claim was actually given to "Acme Corp", ie. it was boilerplate cut & pasted from the MIT example, then you could almost claim that there is NO copyright on this work.  Acme Corp is a fictitious organization, and failing to update the boilerplate puts the claim on dubious grounds.

But let's be good citizens, and grant the copyright to the actual claimants.


Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:


This next section, the Permission notice, is stating that you can do just about anything you want with the code, including modifying the licensing agreement!  The catch is that you can't change the license on the existing code - you can change only what you modify.


The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.


This part simply means you have to persist the notices that are above.

So, what can you do?  


You can and should lay copyright claim to the code you wrote and / or modified.

To do so with the same MIT license:  Just add your copyright notice after the 2012 Acme Corp copyright notice in the files you modified.  

You can license your modifications under a different license, if you so choose. 

To use a different license:  Add your copyright and license notice after the entire 2012 Acme Corp block (copyright, permission / license, exclusion of warranty) in the files you modified.  



In the simplified case of your question, here's what you need to do:
Emphasis added to highlight the differences.


Original work Copyright (c) 2012 [Acme Corp]  
Modified work Copyright 2012 Steve Bennett  

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

