Timesheets are sometimes entered against the wrong stage. Sometimes on the wrong project altogether!
Sometimes You will need to perform a WIP transfer to move all the timesheets into a temporary stage, then change the rate group and then move all the timesheets back.
Go to Project > Financials > Transactions
Change the view from 'Default' to 'WIP'
Select all the transactions that belong in the same stage.
I strongly recommend doing the following one stage at a time. If you move all WIP into the temporary stage all at one time, you will not know which stage the transactions originally belonged to.
After you have selected your transactions, scroll to the bottom of the page and click the down arrow in the grey box and select 'WIP transfer'
Select the correct stage in the Stage drop down then Save.
If the transactions have been entered against the wrong project altogether, select the correct project in the Project drop down.
You will be returned to the Transactions page, with all selected WIP in the correct stage.