How do i purchase more storage

You cannot upgrade your internal storage. You can however upgrade to iCloud+. See Upgrade to iCloud+ and manage your subscription – Apple Support (UK)

Hello ~ You can buy more iCloud storage not device storage …click below to review…

Upgrade to iCloud+ - Apple Support

~Katana-San~

You can't. iPhone internal storage is a fixed size and can't be increased.

Here are some helpful articles that can guide you on how to free up storage space on your phone.

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

How to check the storage on your iPhone and iPad - Apple Support

Manage your photo and video storage - Apple Support

Manage your photo and video storage - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

Clear Other storage on your iPhone or iPad - Apple Support