How can free up space on my iPhone?

1.

You cannot expand the storage capacity of your iPhone.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

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

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

2. 

If you wish, you can upgrade the 5 GB free iCloud.

See -> What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

See also the following Apple articles:

Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

How to use iCloud on iPhone -> How to use iCloud on iPhone – Apple Support (UK)

Manage your iCloud storage -> Manage your iCloud storage - Apple Support

Read this support article for how to check the storage on your iPhone, iPad, and iPod touch

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

- General article on seeing what is taking up storage on an Apple mobile device and tips on how to clear some storage space.

How to Free up space on your iPhone [Apple YouTube video] --> https://www.youtube.com/watch/bnhuHSv_WrI

Note there's different kinds of "storage". Click on this link to learn the difference between your device and iCloud storage, ways to manage your space, and upgrading to iCloud+ if you need more --> What's the difference between device storage and iCloud storage? - Apple Support

Read this support article to learn how to how to check the storage on your iPhone and iPad, and how to utilize iCloud to optimize the storage on a device --> How to check the storage on your iPhone and iPad - Apple Support

my Phone is I phone 6s and storage is 16GB but there is no enough app on my phone and my phone is full y can I buy some space

Delete unusefull app on ur phone