Transactions should be editable unless your GnuCash file has "Book Options" set which make transactions read-only after a certain number of days.
To check this, open up File/Properties. Then on the Accounts tab, look at the setting for "Day threshold for Read-Only Transactions (red line)". Once this value is set to "0", you should be able to edit your existing transaction split to show the intended expense account.
Alternatively, you could record a new adjustment transaction that debits the 'Office Supplies' account and credits the balance in 'Miscellaneous'. This provides a fuller audit trail and may be preferable if you've already provided an expenses report to management (or club committee, etc) showing the expense recorded as 'miscellaneous'.