If you have a lot of important things you don't want to lose I hope you backup regularly. Your only safe guard against lost data, by any cause, is a backup. Ordinarily an iOS update won't cause problems, but it might, and there is no way anyone here can say it will or won't - we cannot possibly know if it will in your specific case. Or you could drop and break the phone, or get it wet, or any one of a thousand ways a device might get damaged or altered and data lost.
Seriously, without a backup, sooner or later you WILL lose data, it happens all the time for countless reasons. So if it truly matters to you, figure out a way to make a backup. You can always backup to iCloud.
Apple, like any and every software company on the planet, assumes absolutely zero responsibility for your data on your devices.