Radix 7.x-3.1 uses Gulp and no longer requires Compass gems. The update has to be done manually.

Step 1: Update Radix

1. Download Radix 7.x-3.1.
2. Clear your site cache. drush cc all

Step 2: Remove unused files

Delete the following files from your subtheme:

1. config.rb
2. Gemfile
3. Gemfile.lock
4. Guarfile

Step 3: Copy required files.

Copy the following files from the default kit to your subtheme.

1. radix/kits/default/bower.json to /path/to/nameofyoursubtheme/bower.json
2. radix/kits/default/config.json to /path/to/nameofyoursubtheme/config.json
3. radix/kits/default/package.json to /path/to/nameofyoursubtheme/package.json
4. radix/kits/default/gulpfile.js to /path/to/nameofyoursubtheme/gulpfile.js

Step 4: Update values.

Update the following values in bower.json, config.json and package.json:

1. {{Name}} : name of your subtheme.
2. {{Description}} : description of your subtheme.
3. {{machine_name}} : the machine name of your subtheme.

Step 5: Fix SCSS files and directories

1. Move /path/to/nameofyoursubtheme/assets/sass to /path/to/nameofyoursubtheme/scss.
2. Delete /path/to/nameofyoursubtheme/scss/screen.scss
3. Copy radix/kits/default/scss/default.style.scss to /path/to/nameofyoursubtheme/scss/nameofyoursubtheme.style.scss
4. Copy radix/kits/default/scss/radix to /path/to/nameofyoursubtheme/scss/radix
5. Rename /path/to/nameofyoursubtheme/scss/global to /path/to/nameofyoursubtheme/scss/base
6. Rename /path/to/nameofyoursubtheme/scss/partials to /path/to/nameofyoursubtheme/scss/components
7. Rename /path/to/nameofyoursubtheme/scss/base/_helper.scss to /path/to/nameofyoursubtheme/scss/base/_helpers.scss
8. Rename /path/to/nameofyoursubtheme/scss/base/_mixin.scss to /path/to/nameofyoursubtheme/scss/base/_mixins.scss
9. Rename /path/to/nameofyoursubtheme/scss/base/_variable.scss to /path/to/nameofyoursubtheme/scss/base/_variables.scss

Step 6: Fix JS files and directories

1. Move /path/to/nameofyoursubtheme/assets/javascripts to /path/to/nameofyoursubtheme/js
2. Rename /path/to/nameofyoursubtheme/js/script.js to /path/to/nameofyoursubtheme/js/nameofyoursubtheme.script.js

Step 7: Update info file

Edit nameofyoursubtheme.info and update the following lines:

1. stylesheets[all][] = assets/stylesheets/screen.css to stylesheets[all][] = assets/css/nameofyoursubtheme.style.css
2. scripts[] = assets/javascripts/script.js to scripts[] = assets/js/nameofyoursubtheme.script.js

Step 7: Install bower and npm modules.

Run npm run setup

Step 8: Run gulp

Run gulp to start watching for changes.