Hoi, Dui! I s'pose I'm probly th' closest thing we got to a Anvil Expert (all that time spent hammerin out ingots fer jewelry, y' know) so I'll try t' answer ye in ways I hope'll be comprehensible.
Anvil Studio's got a pretty steep learnin curve, an ye're right, there don't seem t' be a good manual anywhere. Took me probly 5 or 6 months o' trial an error t' get reas'nably comfterble with it. There ARE instructions in th' blue portion o' th' window under th' tools, but they ain't always clear...
SO anyways, t' shorten (truncate) a tune, place yer cursor at th' spot where y' want it t' end (or start, in case ye'd like t' delete th' annoyin tempo drumbeats that so many midis seem t' think is necessary) an then go t' File > Truncate Song, which gives ye two options, an then a box t' decide if ye're countin measures, quarter notes, eighth notes or MIDI ticks. It's pretty self-explanatory once ye've found th' Truncate Song thing.
T' change a note's pitch, there's sev'ral ways: first off, y' can place yer cursor on th' note (th' cross shape gets little arrows on its ends) an then L-click an drag it up an down th' staff. Best t' be zoomed in a bit t' do this, as it's easy t' drag farther than y' wanted to. Y" can also place th' cursor just t' th' left o' th' note an use th' up an down arrow keys t' move it, but it will only use staff intervals—can't move a half step this way if it's a whole step on th' staff. Also, if it's a note in a chord, this method moves th' entire chord. Third thing y' can do is R-click on th' note an choose Properties, which opens a box with all th' MIDI parameters o' that particular note, an y' can change th' pitch from a drop down menu.
If y' get th' hankerin t' ADD a note or two, make sure that th' Insert Mode box is unchecked (bottom of th' tools panel, near th' left edge) or else every note y' put in pushes th' rest o' th' tune t' th' right.
Lots more I could tell ye, but it'll be simpler if I just try t' answer questions y' may come up with, rather than give a full out tutorial here...