sed -i '1s/^xEFxBBxBF//' orig.txt
sed '1s/^xEFxBBxBF//' < orig.txt > new.txt
sed $'1s/xefxbbxbf//' < orig.txt > new.txt
sed '1s/^xEFxBBxBF//' < orig.txt > new.txt
You can also overwrite the existing file with the -i option:
sed -i '1s/^xEFxBBxBF//' orig.txt
If you are using the BSD version of sed (eg macOS) then you need to have bash do the escaping:
sed $'1s/xefxbbxbf//' < orig.txt > new.txt