need help adding if modified since tag to my current .htaccess for proper caching

have set up my .htaccess file using snippets of code from html5's boilerplate. however, if I update the content of lets say a css file, it doesn't update. I have to delete the actual cookie from my web browser to see changes. could you help me implement something with like: if-modified-since so that the browser would be able to tell that a file has changed and dispaly it

Here is my current .htaccess file:

# ------------------------------------------------------------------------------
# | ETag removal                                                               |
# ------------------------------------------------------------------------------

# Since we're sending far-future expires headers (see below), ETags can
# be removed: http://developer.yahoo.com/performance/rules.html#etags.

# `FileETag None` is not enough for every server.
<IfModule mod_headers.c>
Header unset ETag
</IfModule>

FileETag None

# ------------------------------------------------------------------------------
# | Expires headers (for better cache control)                                 |
# ------------------------------------------------------------------------------

# The following expires headers are set pretty far in the future. If you don't
# control versioning with filename-based cache busting, consider lowering the
# cache time for resources like CSS and JS to something like 1 week.

<IfModule mod_expires.c>

ExpiresActive on
ExpiresDefault                                      "access plus 1 month"

# CSS
ExpiresByType text/css                              "access plus 1 year"

# Data interchange
ExpiresByType application/json                      "access plus 0 seconds"
ExpiresByType application/xml                       "access plus 0 seconds"
ExpiresByType text/xml                              "access plus 0 seconds"

# Favicon (cannot be renamed!) and cursor images
ExpiresByType image/x-icon                          "access plus 1 week"

# HTML components (HTCs)
ExpiresByType text/x-component                      "access plus 1 month"

# HTML
ExpiresByType text/html                             "access plus 0 seconds"

# JavaScript
ExpiresByType application/javascript                "access plus 1 year"

# Manifest files
ExpiresByType application/x-web-app-manifest+json   "access plus 0 seconds"
ExpiresByType text/cache-manifest                   "access plus 0 seconds"

# Media
ExpiresByType audio/ogg                             "access plus 1 month"
ExpiresByType image/gif                             "access plus 1 month"
ExpiresByType image/jpeg                            "access plus 1 month"
ExpiresByType image/png                             "access plus 1 month"
ExpiresByType video/mp4                             "access plus 1 month"
ExpiresByType video/ogg                             "access plus 1 month"
ExpiresByType video/webm                            "access plus 1 month"

# Web feeds
ExpiresByType application/atom+xml                  "access plus 1 hour"
ExpiresByType application/rss+xml                   "access plus 1 hour"

# Web fonts
ExpiresByType application/font-woff                 "access plus 1 month"
ExpiresByType application/vnd.ms-fontobject         "access plus 1 month"
ExpiresByType application/x-font-ttf                "access plus 1 month"
ExpiresByType font/opentype                         "access plus 1 month"
ExpiresByType image/svg+xml                         "access plus 1 month"

</IfModule>

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