replace word for particular variable using windows batch or powershell script

I have a text file for setting up environment variables. I need to change the value for 'config' variable. below is the file content.

env.mode: on

config: env

server: 2

db : mssql

.........

.........

env value can be anything. it should be replaced to prod. (config: prod). I know we can replace the word using powershell get-content. since word is not static, need to set the value. using Powershell or windows batch script..Can someone help me?