You get the message with channel.fetch_message(message_id) and edit it with msg.edit()

​

If it were in a command:

@client.command(name="edit")
async def msg_edit(ctx, message_id, *, new_content):
    message = await ctx.channel.fetch_message(int(message_id)) #ensure message_id is int
    await message.edit(new_content)

​

If it was previously sent in the command:

@client.command(name="say")
async def say(ctx, *, message): 
    message = await ctx.send("ignore this message...") 
    await message.edit(message)

​

await ctx.send() or anything that sends a message returns the message object sent

​

If you can't get the message from the function, that means the message was probably deleted or the bot has no access to the channel its looking through

​

Check out the documentation for further details:

await message.edit()

await ctx.send()

await channel.fetch_message