You'll need 2 things: your save file and a save file editor program. The save file has to be set up in a particular way, which I will describe below.

In order for this to work, you have to change/raise your stat in order for it to be registered in the save file. Let’s say you want to hack fitness. When scheduling your week, put at least one day of working out. If you want to hack your charisma, put in at least one day of blogging, etc. Once your desirable stat has risen (become higher than 5), save and remember the save slot. Close the game.

Your save file location depends on the directory in which you've installed the game. To find it, right click on the game in your Steam library and choose “Properties”. Next, go to Local Files tab and click on “Browse Local Files”:
From there, click on “game”, then “saves”. You should see save files with names like:
The first number corresponds to the save page and the second number corresponds to the save slot.

Using an online save editor is the easiest, such as saveeditonline.com. Upload the save file from step 1 and if you did everything right, you should see your stats like this:
Here is how each stat is called:

Money = money
Intelligence = intelligence
Fitness = physical
Fashion = fashion
Charisma = charisma
Popularity = reputation
(note: it might take a few days of blogging before your popularity rises)

I haven’t been able to find GPA, but it’s possible it’s in there, too.

Edit them to the desirable values, 300 is max for stats. Not sure what the max is for money, but you don’t need much anyway. I’ve bought everything in-game and it cost me less than 10,000. Once you’re done, scroll to the bottom and save your edited file. It will be downloaded to your PC. Now you can replace your original save file with the edited one.

Avoid getting your fatigue higher than 30, as Sian might be forced to rest and you’ll lose stat points. Sleeping during the week also lowers your stats. If you’re tired, either sleep on the weekend, or go to Pho 24 during the week to eat. Happy editing.