I booked three for my arrival day in May and ended up only using one, I didn't need to cancel the remainders as they are not valid once the day is ended. None of my other FP+ for the trip were affected.
You can change the times or experiences of FP+ right up until they time of each alloted one, for example if you had a FP+ for Everest at 11.30am and you couldn't get there in the alloted hour or you didn't feel like doing it then you can either change it for a later time (if one is available) or for another experience (again, if available) if you find nothing that fits you can keep the original one, it does talk you through the steps involved on the app. If you haven't used any or all the FP+ and you are in a different park you can change that as well. You will need to cancel any unused ones to get your extra FP+, again a simple procedure.
If you have any doubts about anything then just head to the well marked kiosk, they are plentiful in all the parks and the CMs I dealt with were all excellent.
My gripe is that people are making FP+ to be more complicated than it is. They cost nothing and be flexible if you want to change them and if you don't use them don't panic, unused ones will not affect FP+ on other days. I booked all mine before my trip and most days I ended up changing them at some point, 80% of the time I got what I wanted even at very short notice, but, if you going at a busy time it would be a lot less. BTW, stupid here had a FP+ for Anna and Elsa and not realising how busy it was deleted it, the next day I changed my mind and you know the rest.
My big advice is go for morning ones when you get the options, because once they are used you can get another one per hour, so, if you used all of them by 11am you can get an extra one every hour no matter if you hop parks in the meantime. I believe you do need to be in the park itself, but, I got a Soarin' FP+ extra at 5pm for 5.45pm, impossible when it was paper FP.