You can use sed to change all of the strings in a file. Put your sed command in a for loop, such as:
for file in *.txt; do
sed -i -f sedprogram "$file"
Make sure that you test the sed program thoroughly on a copy of a few of the files before running it on all of the files. For example, if you are replacing a phrase, it might be split between two lines. The before string might be a substring in places that you want to keep. If the first word of a phrase might begin a sentence, then the string and the replacement need to be capitalized.