I need to know if I can do a system update on my jailbroken iPhone 3GS?

I didn't jailbreak my phone myself someone else previously owned and jailbroke my phone. But I want to update to the newest version of the apple software but when I try it doesn't complete the update. And I also don't have a computer.
I'm on straight talks network and I had to change apn setting for my sim to work and i'm worried if I unjailbroke my iPhone it'll mess up my sim. I've searched the web for information on it but never found anything that told me what I was really looking for.

When you jailbreak an iDevice the added files are all being applied to teh OS, this is a whole different field of memory that you can't even touch naturally when you perform a system update or restore you are wiping your entire OS and installing another new OS or using a OS that hasn't been touched

System Update will delete anything pertaining to a jailbreak including but not limited to, all of your games that t you may have installed with installous, any roms or emulators all custom themes and anything else downloaded from cydia

In shorter terms a system update will get rid of your jailbreak but you can still perform one, if you wish to have jailbreak alter you will still have to re jailbreak the device

If you really need to update then restore the iPhone this requires a computer and possibly an employee at best buy or something could do it, hell use one of the macs on display in an apple store or best buy.

I would recommend investing in a computer >.>