how I can raise my storage capacity to 128 GB?

Buy a phone with greater capacity. An iPhone will never have more storage than it did when you bought it.

You can do a few things to reduce the amount of storage you are using. There are some tips in this distantly related article→If you need more space for an update - Apple Support

You can delete stuff, you can enable Optimize photo storage. That’s about it, unless your Other category is very large (over 5 GB). For that, back up your phone, then restore the backup.