The update is available for download from m.opera.com in your Symbian web browser, and will shortly also be available in the Nokia Store. Rather curiously, the update announces itself as a download with version number '11_50'. Still, treat whatever is offered by m.opera.com as the latest and greatest.
From the Opera blog post:
Opera Mobile 11.5.1 Symbian: change log
- Added predictive text input and language choice in search field
- Improved stability in out of memory situations
- Fixed issues with browser freezing
- Fixed issues with browser when Opera Link is enabled
- Fixed issues with text selection and cut/copy/paste
- Fixed issues with reappearing search engines after deletion
- Fixed issues with selecting text using Up/Down navigation keys
- Fixed issues with third-party input keyboards (QQ, Baidu, Sougou)