- At first, replace the old Bootstrap files with the latest ones. Here you can download the [latest version] (http://getbootstrap.com)
- Check if there are any errors in the *.less files in the theme's folder, and in the base theme's folder too.
- Typical error after upgrading is when a mixin gets "undeclared". Then you have to import the mixins file.
- Recompile the *.less files of the theme (WEB ESSENTIALS/Re-compile All LESS Files).
- After recompiling, check if there are any compile errors.
- Compare the original look of the site with the look of the upgraded one.
- If there is anything different (text size, color, margins, paddings, etc), fix them by modifying the *.less files.