While it isn't too difficult for someone who knows how to work on bikes, it may be beyond the reach of someone who has never worked on a bike before.
You'd have to:
- Remove the grips
- Disconnect the shift cables
- Remove the grip shifters, pulling the cables out with them
- Buy the correct thumb shifters
- Install the thumb shifters, using the same housing if it's brand new. (Shifters usually come with new cables.)
- Make sure the housing is mounted and seated correctly
- Connect the cables to the correct derailleurs
- Adjust the tension of the cables so that the derailleurs shift correctly
- Install grips.
Do you know how to fix a derailleur if it doesn't shift properly? If the answer is "No" then you'll probably have significant trouble with this project.