Find all files in the current directory (denoted by ~/*) with a ".txt" extension and execute the command "sed -i 's/[0-9]\{2\}/&-/g' {} +" on each of them. The command "sed -i 's/[0-9]\{2\}/&-/g' {} +" will replace any two consecutive digits with the same two digits followed by a hyphen.
find ~/* -name "*.txt" -exec sed -i 's/[0-9]\{2\}/&-/g' {} +