how do I upgrade browser for new MSN?

what do you mean

msn is short for microsoft network or?

pretty sure they site works on both safari and chorme on my imac

This is what it shows when opening MSN:You are using an outdated browser. Please upgrade to the latest version for the best MSN experience.

Microsoft is playing games with you. Your current Safari browser is just fine.

Microsoft does not offer a current version of Internet Explorer for OS X, so there is no 'upgrade' available from them.

If you REALLY WANT to download and install a different browser, the current release of Firefox 32.0.3 DOES correctly display the MSN.com homepage as of today, October 4, 2014 around 11:00 p.m. EDT in the US. No guarantees about tomorrow, next week, next month, next year...

How does MSN KNOW what browser you are using, and why do they care?

When you connect to MSN.com (or any other website) the site can query your browser to determine which operating system, which browser and which browser version you are using when you connect to MSN.com

MSN "sees" that you are not running a recent version of Internet Explorer, (MSN IS a part of Microsoft, maker of Windows, of course, so in their ideal world, EVERYONE should use Windows!) and it displays the warning message and displays the page without much formatting, so that what was the 'pretty' MSN home page now appears as a single column of somewhat jumbled headlines and photos, just as it might on an older version of Internet Explorer.

If you want to make the message go away and restore the 'pretty' appearance to the MSN homepage, you can follow the steps listed in the link below to temporarily 'trick' MSN into thinking that you are using Microsoft Internet Explorer 9 or Internet Explorer 10.

Msn says I must update my browser

I confirmed yesterday (October 13, 2014) that MSN home pages no longer 'discriminate' against the most recent Safari releases available for OS X 10.6 Snow Leopard, OS X 10.8 Mountain Lion, and OS X 10.9 Mavericks.

Probably also true of OS X 10.7 Lion, but I don't own a copy of that.

The most recent Safari versions now display a readable, properly-formatted MSN home page with multiple images in several columns, instead of the single-column poorly-formatted mess that also produced the 'outdated browser' message.

In other words, it was "them", not "us".