For this task you have to add these two lines to your crontab file:
30 8 * * 1 amulecmd -c "setdownbwlimit 2"
15 21 * * 4 amulecmd -c "setdownbwlimit "
Note, that you have to use "crontab -e" to edit your crontab file.
You also have to have cron installed and running.
For more information, see "man 5 crontab" and "man amulecmd".