How can I upgrade my bb to 6.15.00 from 15.16.08 iOS 6.1.2 on iphone 3GS(jailbroken)? HELP ME PLEASE

Apple has released iOS 6.1.3; do not upgrade or restore (if you want to keep your jailbreak). You cannot downgrade/upgrade/restore to 6.1.2 using SHSH blobs on newer devices; older devices that can use SHSH blobs are iPhone 4, iPhone 3GS, and iPod touch 4th gen. Learn more.

Jailbreaking iOS 6.x instructions. Official website: evasi0n. News: @evad3rs, @pod2g, @planetbeing, @pimskeks, @MuscleNerd.
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Source: http://www.jailbreakqa.com/questions/174831/how-can-i-upgrade-my-bb-to-61500-from-151608-ios-612-on-iphone-3gsjailbroken-help-me-please

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