There are 2 possibilities to determine an upbeat. The first is mainly for newly written scores. Select the note or rest after the requested upbeat:

Choose:
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NtEd changes the begin such a way, that the selected chord or rest is the first member of the first full measure:

The 2nd procedure is mainly for scores with beginning rests. Select the note or rest which shall be the first element:

Choose:
→ →
NtEd delets all before the selected element. Thus, the selected element becomes the first element.
