Any expense that's already included in a landlord statement will be locked from editing, to avoid you having reported incorrectly to the landlord.
This will be indicated at the bottom bar of the expense window as below:
If you entered some expense details incorrectly (not the amount), then you can just delete the statement, correct the expense, and recreate the statement. The total amount of the recreated statement shouldn't be affected.
(Please note, you can only delete the most recent statement for each landlord).
If you entered an incorrect amount in the expense, you wouldn't want to change it because this will affect the total of the statement which you've already sent to the landlord.
In this case, you should create a credit note or another invoice with the amount difference to rectify the old incorrect invoice.