Right-clicking (CTRL-clicking for Macs) one or more selected transactions will bring up a context menu providing functions that can be applied to the selected transaction(s). One of these functions is Batch Change, which makes changes to the currently selected transaction(s). Batch Change allows you to make changes to the Description, Check#, Memo, Tags, Category, and Account fields in the register.
You can highlight multiple transactions by either Shift + clicking (click the first transaction you want to change, then hold shift and select the last one, thereby selecting all the transactions between the two clicked) or Ctrl + clicking (select multiple transactions by holding Ctrl and highlighting or unhighlighting the transactions). he two clicking methods can be combined, by the way, so if you have a very large block to go to a new account with one or a few transactions to stay, you can Shift click the large block to highlight them and then Ctrl click the ones you want to unhighlight.
Changing the Category of a transaction changes the information which is currently in the Category field of the transaction. Changing the Account of the transactions will move them out of the current account (or category) register into a different account (or category) register.