@pderbogosian as long as the time activities haven’t been released you should be able to put the time activity on hold and make changes.  there are a few ways to do this but if you go to the weekly crew entry, select the work group and week, you’ll see a check box with a column name of “hold” 

if you click that, it will open the activity to make changes: 

you can also do this via employee time activities at the employee level, however you might need to click save for the system to recognize the activity is open again

Hope this helps!