I can’t download IOS version 15.5 on my iphone 6. What can I do?

It is not possible, iPhone 6 is an outdated iPhone (9 september 2014), can’t be update to a higher iOS.

iOS 12.5.5 is last security updates for iPhone 6 -> released 23 september 2021.

 Considering a newer type of iPhone?

rinko ~ Apple offers many newer iPhone models which will run iOS 15:

iOS 15 and iPadOS 15 are compatible with these devices

...found by searching Apple's Knowledge Base.

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Update wise, there is nothing that you can do if you have an iPhone 6. It cannot be updated to iOS 15.x.