how do I upgrade to 200GB a month?

See the following from Upgrade to iCloud+ - Apple Support

Also note that the upgrade is not "200GB a month" -- you pay monthly but only get a total of 200GB; it does not increase monthly.

How to upgrade on your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan and follow the onscreen instructions.

If you're on your iPhone, iPad, or iPod touch with iOS 14 or iPadOS 14 or later, tap here to upgrade to iCloud+. 

How to upgrade on your Mac

1. Choose Apple menu  > System Preferences, then click Apple ID.
2. Click iCloud.
3. Click Manage in the lower-right corner.
4. Click Change Storage Plan or Buy More Storage, and choose a plan.
5. Click Next and enter your Apple ID password.

How to upgrade on your Windows PC

1. Open iCloud for Windows.
2. Click Storage.
3. Click Change Storage Plan.
4. Choose a plan, then click Next.
5. Enter your Apple ID password, then click Buy.