To manually clean cache you should access to your server with a ftp client, like Filezilla.
Or you could do it with your hosting file manager...
But if you have never done it I think it's better if you ask your friend.
If you want to do it inside MODX actually you can: you have to use tab "FILES" top-left corner, then look for core/cache and inside cache remove directory.
But, again, if you have never done anything like this, I
REALLY SUGGEST you to ask your friend

if you delete the wrong file you can "break" your site!!