Go to Settings, choose Safari, and then tap on Clear Cookies and Data. That’s all.
Go to Settings, General, Usage, and choose Safari. In the top right corner, you’ll see the Edit button. Click on it, and you will see the Offline reading list. Tap delete right next to it.
If you keep a bunch of videos, photos and music, here is a question: Do you really need all of that? No, you don’t. Backup the files, and then remove them from your device.
You probably have many saved conversations on your device. Delete them, at least those with attachments such as videos, and photos.
You certainly don’t use all the apps, so make sure to delete the ones you don’t need. Find the app’s icon on the Home screen, press and hold until you see an X. Click on it, and the app will be deleted.