This paper deals with formal specification and verification of Java bytecode update. Programs update for java applications has gained a wide interest since it is used for several purposes: transforming semantics of a program, adding features to a program or performing optimizations. In this paper, we focus on program transformations for java programs at the bytecode level. Because these transformations may introduce errors, our goal is to provide a formal way to verify the update and establish its correctness. Our approach for formal specification and verification of updated Java bytecode programs is based on four ingredients: a formal interpretation of the semantics of update operations, a functional representation of bytecode, bytecode annotation and predicate transformation calculus. We use the concept of Hoare predicate transformation to derive a specification of an annotated bytecode. Annotations are used to express update operations within the code. A functional representation is used to model annotations and bytecode. The approach derives then a new specification for the annotated bytecode using a weakest precondition calculus defined to deal with update operations. Verification conditions are then generated and proved to establish the correction of the update.