How do I cancel 50GB and add 500GB ?

You don't need to worry about canceling. Choosing the new storage amount will "prorate" based on the amount you already paid. Just upgrade to the level you want and you are good to go.

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.
4. Tap Buy 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 your iCloud storage. 

How to upgrade on your Mac

1. Choose Apple menu  > System Preferences, then click Apple ID. If you’re using macOS Mojave or earlier, you don’t need to 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.

https://support.apple.com/en-us/HT201318