Find and replace text in a very large text file with in folder

using powershell Find and replace text in a very large number text file 

i have 3 folders like folder1,folder2,folder3,with inthe 3rd folder i have a 3 text files like pw1.txt, pw2.txt, pw3.txt using powershell we can find the largenst text file number find and replace the text to largest file number 

for ex:we have pw3.txt this is a big number pw3.txt it will find and replace the test on the pw3.txt