Unable to update my iPhone

As the message says you have to make more storage available by deleing less used items. You can try moving them to a computer or cloud storage. The iPhone update requires a certain amount of free space. This article may help:

Update your iPhone or iPad - Apple Support

1) The Wi-Fi signal is too weak.

2) You need to charge the battery to 100%.

3) There is not enough room to install the new update. You need to delete apps, photos, music, etc that you no longer need to make room for the update.

It might help to use a PC or Mac for the update as well.