Lately I had to give crash-course in music scores writing program Sibelus (version 7) and after that course I got telephone calls twice asking and precising some of very basic common tasks in the software. Despite I got the main problem - how to put an upbeat, I ended up struggling about how far and what exactly my student was doing working with Sibelius, because there are two ways how to do it.
Ok, so much for introduction, here go my solutions:
1) Easy way. Creating upbeat at quick start. When score writer has chosen what type of writer want to create, next thing is to choose different properties for scores, like time signature, tempo, key, etc. Under the Time signature setup there is option to create Pick-up bar with different length than it is defined by time signature. Easy as pie, but often forgettable.
2) Not so easy way. But, if score writer forgot to create upbeat at quick start, then it is still possible to make upbeat later. Here, writer have to select "Notations" tab to access "Time signature" dropdown selector. In dropdown selector there are common signatures, and under them there is button "More Options". And upbeat creation can be found there. Checkbox at "start with bar of length", choose length and press OK. After such manipulation mouse cursor becomes active and now it will create a bar with previously defined length in the place where score writer will click. So, because upbeat needs to be in the beginning of scores, score writer must click somewhere on the first bar or, better, on the key.