How do I clean out the Cache in my iPhone?

Catrina ~ Learn how to delete your history, cookies, and cache in Settings:

Clear the history and cookies from Safari on your iPhone...

Also turn your iPhone off & then on again:

Turn iPhone on or off - Apple Support

If your iPhone 7 is slow or glitching, does it still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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If by "cache" you are referring to the iPhone's DNS cache, there are two methods to clear it:

1. Toggle Airplane Mode on/off.
2. Reset the Network settings. Tap on Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings