In general, you can change descriptive information of Time+Expense items anytime (if you have write access) - example: change Project, Activity type pr description.

The approval protects Time (hours) and Expenses (receipt amount) to be changed after it is approved.

So to change submitted or approved reports, you have the following options:

Before approval, just recall the submitted report. 

You can then go to the Timesheet and query the report and update it.

You could also delete the report (often prevented as that looses history).

See the editing options below.

When done, just submit the report.

After approval, you need to be an administrator - go to the report and set the Status field to New and save the record.

See the editing options below.

When done, set the status back to Approved.

Editing Reports

You can change reports directly via the related list or via Timesheet.

Change T+E Items directly

In the related list, select the record you want to change. To change the hours, change the Timesheet Details (Day 1/2/3/..) NOT the Hours field at the top (that is recalculated from the Timesheet Details):

Save the record and either re-submit or set the status to e.g. Approved.

Change in Timesheet

You can view submitted reports in the timesheet and also update it, if the report is rejected or set to New.

1) in the timesheet, select the week and click Query records for week

2) if there are T+E Reports for that week, select it from the dropdown and Query records for week again. You will get a message if you can modify the report (or not).

After you saved the changes, just click Go to Report to re-submit the report for approval.