How can I change an eight rest in the middle of an piece to an sixteenth?
There are several ways - here’s one.
Press Return to enable note entry, then I to enable insert mode. Press 5 to select eighth note, then Delete to delete the rest. Press 4 to select 16th note, then comma (,) to enter the rest.
…or don’t delete it at all. Just enable insert mode, select the rest, press 4 (or the 16th duration button).