You can delete accounts and transfer any of the past transactions associated with it to another account! Please go to Delete/Hide Accounts in Accounts Settings.
Delete an account:
1. Menu
2. Settings
3. Accounts Settings
4. Tap Delete Account
5. Tap trash can in the upper righthand corner
6. Select the account to which you wish to transfer associated transactions (or leave the transactions as Uncategorized)

Hide an account:
1. Menu
2. Settings
3. Accounts Settings
4. Select the account you wish to hide
5. Turn off the Display Account setting
6. You will no longer see the account on your expense tracker, analytics, or regular expenses pages