How can i download ios 15.8 in iphone 6plus

You don’t. that requires a 6s or 6s plus. The 6 plus can not run anything later than ios 12.

An iPhone 6 Plus cannot update past iOS 12. You need a new phone.

You start by reading the previous posts that explain why you can't.

PS. there is no such model as an iPhone 6a

How can I download iOS 15.8 iPhone 6 a

Ok

This has already been answered above.