Although this example applies to editing a path in a specific file, many directive values in config files requiring modification with sed can be encased between a pair of quotes. Feeling that the solution could be of broad interest, I've shared it below:
sed 's#"$#:/opt/dell/dcc"#' /etc/environment
The benefit of matching on the final quote is that it ensures even if the path changes in future, this sed expression will still work correctly appending the custom path on the end of it before the closing double quote.
The above sed expression is tested and known to work correctly, but others might have a more elegant way to achieve the same result; please feel free to share them here.
Don't forget to add
-i switch to make changes persistent after testing-