how do I update Microsoft office to 64 bit

Get your wallet out. The only current 64-bit application suites supported by Microsoft are either the subscription Microsoft 365 or the single-purchase Office 2019 for Mac. The Microsoft 365 applications are available from the Mac App Store, and you can acquire Office 2019 for Mac from the Apple online store.

Your Office 2011 for Mac documents are unaffected by the 64-bit issue.

Plan B might be the free LibreOffice Suite which is a superset clone of MS Office, and it will open all of your Office documents. As with any clone, there may be features present that when shared with other MS Office users, may not appear to them as you had intended, so caveat emptor with features.

None of the alternatives to Office preserve 100% of the formatting of word processing or layout of spreadsheets. If any customizations beyond standard equations or page layout are necessary, you may need to tweak them for use with returning back to Microsoft Office.

For that matter, most formatting options are available freely on http://www.office.com/ freely. But not all.

I'm assuming you were probably on some old version of Microsoft Office and did not verify the upgrade requirements prior to going to Big Sur.

Office365 (subscription) or Office2019 (license) for MacOS is the only option you have to get your Office apps current. There are other non-Microsoft options available but they are never fully-compatible with Office documents, especially when it comes to page-formatting.

Office365 is the best option. It's always updated and kept current. One can find some good deals out there with Office365 subscriptions, and it allows you to install it on multiple machines on one license.

The other option is for you to downgrade back to whatever MacOS level you were on and stay put.

You can't. Office 2019 is 64 bit, and can be purchased from:

https://ps.sale/product-category/mso/

There are two free alternatives for MS Office: LibreOffice and SoftMaker FreeOffice 2018.  Both can open, edit and save Office documents is the MS Office format or their own format.

microsoft have to make an upgrade, they however stopped support for office 2011 and 2016