How can I update my iPhone SE to iOS 15?

Update your iPhone, iPad, or iPod touch

1 - If noting is offered by Apple - then this iPhone is running the latest version of iOS it Qualifies to run.

2 - if #1 above and does not Qualify for iOS - only option is to purchase a new or newer iPhone that does qualify for iOS 15.

Details are required.

Apple markets an awful lot of product. What assistance do you need with what product?

Please take a look at this article:

Writing an effective Apple Support Communities question - Apple Community