My apple phone 10 plus in settings doesn’t say 15 update

First there is no such thing as a 10 plus.

And why can't you update? What is happening when you try?

Do you:

1. Have 6GB of free space?
2. Not using a VPN
3. Have a good WiFi connection?
4. Have a 50% or better battery charge and preferably connected to power?
5. Low Power mode is turned off.

Go to Settings > General > Software Update.

Update your iPhone, iPad, or iPod touch -

https://support.apple.com/en-us/HT204204