In general, you can change descriptive information of Time+Expense items anytime (if you have administrator 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

After submitting, but before approval of the T+E Report, just recall the submitted report. 

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

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

(See the editing options below.)

When done, just submit the report again.

How to Recall in Lightning

In Lightning, go to the Tab / Launcher Item "T&E Reports" and find your report.

In the Report, click on the Related sub-tab and scroll to Approval History

(Your administrator may have moved the related list item to the right column)

The click: Recall

Recall in Lightning

How to Recall in Classic

In the Classic UI, go to the Tab "T&E Reports" and find your report. 

Then scroll down to Approval History. There click on the "Recall Approval Request" button.

Recall in Classic

Recall Approval Setup

To be able to recall a report, make sure that you allow submitters to recall approval requests. 

In Step 6 of the Approval Process:

After Approval

After approval of the T+E Report, 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 (preferred).

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).

You can now

- modify items

- add items e.g. to charge hours to different projects

- delete items

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

Change T+E Items directly

In the related list, select the record you want to change. 

To change the hours, 

change the fields in the Timesheet Details (Day 1/2/3/..) 

NOT the Hours field at the top (which is recalculated from the Timesheet Details):

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