I can't download OS 8.4.1

What happens when you try? Are you trying to do this OTA, or via iTunes?

What error message are you getting. Tell us what you have tried so far.

Try answering the other questions, and what you have done to try and remedy it.

iOS 8.4.1 requires 550 MB of free space to install. Let us know how much free space is on your iPhone when telling us if you are downloading over the air or through iTunes. There is a 100MB limit on OTA downloads and we need to know if this is the problem. To check free space on iPhone Settings > General > Usage and tell us what is under storage there (used, available)

UGADog and others have given you good advice above. You seem to be downloading the update Over the Air which is normally ok but as already mentioned, it has some limitations. You could also try doing it through iTunes instead. Just connect your phone to your computer, open iTunes, select the phone icon on the bar on top, and on the summary page that appears just click on Update. This will bypass any OTA issues.