If the expense item has already been distributed across the months, the amount may stay on the old month when you change the start date. The solution is to set the period again and redistribute the amount.
How to fix it
Go to Setup and Operation and open the expense item by clicking the name or the pencil on the row.
Set the new start and end month in the From / To date field under the General tab, for example July 2026 to May 2028.
Go to the Cost price estimate tab and click the icon to the right of Remaining rev. Cost price estimate. Choose Distribute monthly cost price estimate evenly or Distribute remaining cost price estimate evenly (in your project "cost price estimate" may be called "budget"), so the amount is distributed over the new period.
Save.
If the old figure still shows in the overview, go back to the table and refresh the whole page in the browser.
Good to know
Months outside the new period should be 0. Check this after you have saved.
You don't need to log in again when you refresh the page.