How to update Twitter download
Your best bet is to visit the Bootstrap update page and see what changes affect your implementation.
After replacing the actual bootstrap.css and bootstrap.js files with new ones, many of the changes to your code will be simple. Find and replace the fixes. The update page breaks down what you need to change to update, and it breaks down into sections, so you can only use the parts that matter to you.
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I am creating a project.less file in the same directory as "bootstrap.less". "project.less" includes "bootstrap.less" on the first line. Then I override adding or overriding styles as needed. I am also modifying the variables.less file, but with clear comments on what elements I changed. When I'm ready to create my CSS, I run lessc in the project.less file. For me, this made the transition to 2.1.1 much easier. - I just replaced all files with less downloads, added the styles I changed in "variables.less" and then rerun lessc.
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My best advice is to look over the Bootstrap page and see what has changed. Upload new files and fork your project (if you are using Git) and replace new Bootstrap files in place of old ones. See what the breaks are and try to work from there. I know it sounds tedious, but it might not be a big deal depending on how much of Bootstrap you used in the first place.
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