There is no way to correct the existing timesheet once posted, however it is possible to effectively reverse out the transaction. To do this:
Log into FocalPoint Admin
Click System Options, Time Recording, Timesheet Entry
Ensure the allow negative hours on timesheets check box is selected
Log into FocalPoint web.
Click Timesheets
Enter a timesheet for the same project/cost centre/user/day with the same hours but negative i.e. If the incorrect timesheet is for 3 hours, enter -3.
Post this negative timesheet. The negative timesheet will cancel out the values against the accounts and negate the effect of the incorrect timesheet allowing you to post a correct one.
In FocalPoint enter a timesheet with the correct details.