New iOS on my iphone

Go to---> Settings---> General---> Software-update---> Download and install the update.

You must have a stable Wi-Fi connection.

Or connect to a PC with the latest version of iTunes installed.

Charge the battery to 50% or higher.

Turn off Battery saving mode.

Don't use a Hotspot.

Don't use VPN.

And make sure you have enough free space.

You wrote....... This is m'y first years with iphone.

If you're a new iPhone user..... why do you want to install Beta software????

Doesn't make sense.

Stay away from Beta if you don't know what you are doing.

Stick with the official released update.

Which is iOS 15.3.1

iOS 15.4 isn't even out............ yet.

Your screenshot shows you need to create additional space on the device.

You're welcome.

You need to delete something so that you have enough free space for the update. You can also try updating through your computer.

You need to have at least a 50% battery charge and leave the phone connected after it reaches 50%. Also check to see that you have at least 6GB of free space on your iPhone..

I don't understand why it says iOS 15.4

The public release isn't even out yet.

Good point, I believe they are trying to install a beta Release Candidate.