I am running MAcOs 10.7.5 and want to upgrade to Maverick. Before upgrading I should save the files (47GB) on my Bootcamp. I bought 200GB extra storage on iCloud but how do I save the bootcamp files to the iCloud?.

Saving arbitrary files to iCloud will only be possible with the iCloud Drive as part of the new MacOS X Yosemite, that will be released this fall.

Until then, save your files to an external disk.

See: iCloud - iCloud Drive

iCloud is not for backing up files from your Mac. The following is from this Apple document: iCloud: iCloud storage and backup overview:

What iCloud backs up

You have important information on your iPhone, iPad, and iPod touch. iCloud automatically backs up your device over Wi-Fi every day while it’s turned on, locked, and connected to a power source. With your device information backed up in iCloud, you can easily set up a new device or restore information on one you already have.

Here’s what iCloud backs up:

• Purchased history for music, movies, TV shows, apps, and booksYour iCloud backup includes information about the content you have purchased, but not the purchased content itself. When you restore from an iCloud backup, your purchased content is automatically downloaded from the iTunes Store, App Store, or iBooks Store. Automatic downloading is based on iTunes in the Cloud availability by country, and previous purchases may be unavailable if they have been refunded or are no longer available in the store.
• Photos and videos in your Camera Roll
• Device settings
• App data
• Home screen and app organization
• iMessage, text (SMS), and MMS messages
• Ringtones
• Visual Voicemail

Your iOS device backup only includes data and settings stored on your device. It doesn't include data already stored in iCloud, for example contacts, calendars, bookmarks, mail messages, notes, shared photo streams, My Photo Stream, and documents you save in iCloud using iOS apps and Mac apps.

So follow léonie's advice about getting an external HD.