There are 2 ways that sites can identify a browser:
By User Agent
To modify the user agent that's been sent in http headers, you can use a proxy like Fiddler to intercept and change what IE is sending:
https://www.telerik.com/fiddler
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By feature set
If the site is detecting that you're using an older browser based on it's lack of support for specific features, there's not a lot that you can do.