I spent an afternoon in the pub pressing every button - ooh, what does this do?!
Someone once asked why there's no user guide. The developers said they are doing this as a labour of love, and the rapid development of it means documentation would be out of date pretty quick, so there's no point in writing a user guide.
There is, though, a useful guide if you want to use it with a smart watch:
https://github.com/NightscoutFoundation/xDrip/blob/master/Documentation/WatchGuide.md
The issues page is also worth a periodic look at if any problems crop up - they've sometimes been dealt with there already:
https://github.com/NightscoutFoundation/xDrip/issues
As well as
@porl69 's answer, another way is that after you enter a bolus or carb entry, an "undo" icon appears below the entry/notes icon on the right hand side.
The predictive simulations are incredibly useful but take a bit of getting used to. If I'm undecided about a bolus amount, I'll often enter x units to see roughly how that might pan out, and if I don't like the look of it, I'll undo the entry and see what a different amount looks like.
They're like long range weather forecasts, so need to be taken with a pinch of salt. They'll very often rejig in a completely different way after they get a few more 5 min readings, but once they settle down after about 20 mins, they can be surprisingly accurate.
Have fun with it! I've been using it for about a year now, it makes life a lot easier.
One Q, I've entered an insulin dose, but there doesn't seem to be an option for long-lasting (glargine) so the predictive graph shows me reaching zero glucose in a hour or so 
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