Sure, here's the modified code that will greet the user by their username:

```python
import discord
from discord.ext import commands

bot = commands.Bot(command_prefix='!')

@bot.command()
async def hello(ctx):
    user = ctx.author
    await ctx.send(f'Hello {user.name}!')

bot.run('YOUR_BOT_TOKEN_HERE')
```

In this modified code, we're using the `ctx.author` attribute to get the user who sent the message, and then using their `name` attribute to greet them by their username.

Save the file and run it in your terminal using `python your_file_name.py`.

Now, when a user types `!hello` in a Discord server where your bot is present, it will reply with "Hello [username]!", where [username] is the username of the user who sent the message.
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