# Preview a file edit, via substitution.
sudo sed 's/Name=Xfce Session/Name=Xfce_Session/' /usr/share/xsessions/xfce.desktop

# Replace the same string more than once (appending g flag), via substitution.
sudo sed 's/Name=Xfce Session/Name=Xfce_Session/g' /usr/share/xsessions/xfce.desktop

# Edit a file (adding -i flag), via substitution, in-place; changes are made to the file(s).
sudo sed -i 's/Name=Xfce Session/Name=Xfce_Session/' /usr/share/xsessions/xfce.desktop

# It can become necessary to escape special characters in your string.
sed -i 's/\/path\/to\/somewhere\//\/path\/to\/anotherplace\//' /tmp/filetoedit

# Change your sed delimiter to a pipe to avoid escaping slashes.
sed -i 's|/path/to/somewhere/|/path/to/anotherplace/|' /tmp/filetoedit