In command mode:
This will change all instances ofTHIS in THAT. the % is the line range (% = all), you could also do this: 5,15, which will change occurences in lines 5 to 15 (included).
The g on the end makes sure that all occurences in one line are changed. If you don't use the g, only the first instance will be changed.
Take a look here for a short vi tutorial: vi tutorial
Hope this helps.