if it's a .rar file right click it and select open with... then a second menu should come up with 2 options...Surf the net or choose from an existing program..
Chose the 2nd options and choose notepad, then just just copy and paste the new line where the old line was or insert you new line.Also a common mistake is you have to put the name of the vehicle...
e.g
Taxi, 93,93
DONT FORGET TO BACKUP