Hello and thank you for using JustAnswer.
The battery itself is an easy replacement part, it would not cause you to lose any data or information
You have mentioned that you have an extended warranty with Dick Smith, so I highly suggest that you contact them to have them do the replacement for you. Their number is XXXXX 660 054
If you would like to replace the battery yourself you will have to purchase this battery: http://www.buylaptopbattery.com.au/s/laptop-battery/toshiba/satellite/l750-09j/ and take out your old battery by using the sliding lever (directions will be on the back of your laptop) then inserting the new battery. Its a quick snap in process.
As an pensioner I am finding the charge very severe
The charge from Dick Smith?
Your extended warranty should cover this type of replacement.
Let me know if you have any questions