How to clean out your iPhone's memory: Delete the Safari cache
We're going to start by cleaning out the caches on our iPhone. Note that this will mean you're logged out of any websites you've signed into, though. Follow these steps to clear out the Safari cache:

Press Settings > Safari.
Scroll down and tap Clear History and Website Data.
Tap Clear History and Data.
How to clean out your iPhone's memory: Remove file data
Data stored by other apps can be cleaned using the Usage option in settings. Follow these steps to clean out unwanted data from iOS:

Tap Settings > General > Storage & iCloud Usage
Tap Manage Storage.
Tap an app taking up a large amount of storage.
Take a look at Documents & Data - if there's over 500MB of documents in the app, it may be worth deleting it and reinstalling it to clear the extra space.
Simply tap Delete and head to the App Store to re-download it.

OR use the app cleaner instead