If you're using the Time Approval addon, you can ask for a time entry to be changed before it is approved.

To see your pending approvals, click on the Approvals menu in the main navigation:

Here, you'll see the project and budget against which the time entry was recorded, as well as the name of the person who had submitted the time entry. You can click on the checkmark to approve the time entry:

If you want the time entry to be reviewed, you can request the person who submitted the time entry to change it. Also, you can add the reason why you're asking for a change request:

So what happens if you request a change on a time entry? This is what the person who submitted the time entry will see on their end:

And the time entry in question will be shown like this:

There, they can change the entry, add a reason why, and submit it for approval again:

That's it! Now the time entry will show up for approval again so that it can be reviewed.