A method for updating and formatting a local file is described. An update file, having a newer version of the local file, is accessed from a server over a network and a list of new hash values corresponding to data blocks of the update file is retrieved. Divisions between the data blocks are located when a value of a rolling checksum reaches a particular value, the data blocks being of variable size. New hash values of the update file are compared with local hash values of the local file. For each of the new hash values not having a matching local hash value, the corresponding data block is downloaded from the server and written to a new file. For each of the new hash values having a matching local hash value, the corresponding block is copied from the local file to the new file.