You will use a website called Save Editor (saveeditonline.com).
1. Save your game manually and check the exact time you did it in real life. (This isn't strictly necessary, but it helps you easily identify the file later).
2. Press Win + R on your keyboard, type %appdata%, and hit Enter. This will open a folder called Roaming. Click on AppData in the top address bar to go one folder back.
3. Inside the AppData folder, open the folder named LocalLow.
4. Next, open the folder named IncontinentCell.
5. Then, enter the folder named My Dystopian Robot Girlfriend.
6. Finally, open the folder named Saves.
7. Inside this folder, you will see several game files. Look for the file you saved at the beginning by checking its modification time.
8. Drag and drop that specific file (for example, M1.mdrgslot) into the saveeditonline.com website.
9. A large list of text lines will pop up. Search for the variable called money (you can press Ctrl + F to find it quickly). Click on the green numbers next to it and change it to whatever you want. I set mine to 50 million (50000000). The safe limit is around 99 million.
10. Once you change the money value, scroll all the way down to the bottom of the webpage and click the Download button to save the file.
11. Go to your PC's Downloads folder, copy that newly downloaded file, and paste it back into the game's Saves folder (from Step 6). Click "Replace the file in the destination" when prompted.
That's it! Just open your game, load that specific save slot, and your millions will be right there. (I don't know if there are other ways to do it, but it's the only one I know.)