import ctx #You might need to install this seperately from discord.py (if you don't have it)

# lets say you want to change the bot's prefix...
async def prefix(ctx):

    await ctx.send(f'What do you want to set the prefix to?')

    # wait for responce and lower the text for easy use
    value = await client.wait_for('message')
    value = value.content.lower()

    # this is where you would change the prefix value in a config file or something
    # example: parser.set('prefix', value) <--- the value is your looking for
    
    # prints and messages the output (you don't really need this)
    print(ctx.message.author, 'Changed the prefix to:', value)
    await ctx.send('Prefix changed to: {}'.format(value))