How do I change the extension of multiple files?

Straight from Greg's Wiki:

# Rename all *.txt to *.text
for f in *.txt; do 
    mv -- "$f" "${f%.txt}.text"
done

Also see the entry on why you shouldn't parse ls.

Edit: if you have to use basename, your syntax would be:

for f in *.txt; do
    mv -- "$f" "$(basename -- "$f" .txt).text"
done

Here's how I change all the file extensions in the current directory on Debian.

rename "s/oldExtension/newExtension/" *.txt

(This is the Perl rename command, not the util-linux one. See Why is the rename utility on Debian/Ubuntu different than the one on other distributions, like CentOS?)

A simple command, the rename from util-linux, will do that for you, it replace every occurences of "txt" to "text" in all file matching "*.txt":

rename txt text *.txt