Now that you've backed up your iPhone, you can erase it. Go to Settings > General > Transfer or Reset iPhone. Select "Erase ...
If your iPhone keeps warning that storage is full, there are several settings and system tweaks you can use to reclaim space ...
To upgrade your iCloud on your iPhone, go to Settings > (your name) > iCloud > Manage Account Storage > Buy More Storage. Choose a plan and then follow the instructions. If you upgrade to any paid ...