update 12.4.8

Hi Im trying to update my mothers iPhone 6 to the latest update it wants. i am getting the download to get within a few seconds of downloading but then it stops. the cog emoji continues to turn but nothing happens Ive tried restarting the phone Ive cleared space for the download and Ive deleted and tried to re install the update a number of times with the same result. Im not sure what's going wrong. The wifi connection is not the best but its ok about 5mbps download. Several of my apps have been moved to the cloud as the phone was getting full before I deleted a ton of stuff but now they are waiting or paused. I think none of them will work as they need the new update to allow them to open but I can't think how to stop that from maybe interfering with the download..... any help welcomed