posted by sherif_hashim:

to restore ur iphone while already on 6.15.00 BB:

A) if u used pwnage tool then u ll have to create a /new/ custom 4.1 or 4.2.1 and say /no/ when u are offered the BB update to 6.15.00 since u already have it, then build the custom ipsw and restore to that
B) if u r using redsn0w, then: 
B-1) if u are currently on 4.1, then u can restore to official 4.1 again, and when u see error 1015 pop-up, u kick ur device out of recovery using umbrella, use redsn0w and /don't/ check "ipad BB update" because u already have that :) 
B-2) if u are currently on 4.2.1 then u ll have to restore back to 4.1 (because 4.2.1 will not allow u to kick the phone out of recovery when 1015 occurs due to the silly BB / iOS mismatch check added there) , then do the same steps as B-1, if u want to go again to 4.2.1 your only way is to use a custom 4.2.1 done by pwnage tool in the same steps mentioned in A 

n.b of course saved shsh for the 4.1 ipsw is an important prerequisite for iphone 3gs to restore back to that