I would like to stop the monthly payment of .99$ for 20gb so how do I do that?

Decrease the storage level to the free 5GB. I quote:

You can decrease the amount of storage by selecting a new plan right from your iPhone, iPad, iPod, Mac, or PC (for example, a 20 GB plan to replace a 200 GB plan). The downgrade will take effect after your current monthly or annual subscription ends.

iOS:

1. If you're using iOS 8, go to Settings > iCloud > Storage. If you're using an earlier version of iOS, go to Settings > iCloud > Storage & Backup.
2. Tap Change Storage Plan.
3. Tap Downgrade Options and enter your Apple ID password.
4. Choose a different plan to use when your current storage plan expires.
5. Tap Done.

OS X:

1. Choose Apple menu > System Preferences, then click iCloud.
2. Click Manage in the lower-right corner.
3. Click Change Storage Plan in the upper-right corner.
4. Click Downgrade Options.
5. Enter your Apple ID password, then click Manage.
6. Choose a different plan to use when your current storage plan expires.
7. Click Done.

Windows:

1. Open iCloud for Windows or iCloud Control Panel.
2. Click Storage if you're using iCloud for Windows. If you're using an earlier version of iCloud Control Panel, click Manage.
3. Click Change Storage Plan.
4. Click Downgrade Options.
5. Enter your Apple ID password, then click Manage.
6. Choose a different plan to use when your current storage plan expires.
7. Click Done.