not sure if the trial version actually allows you to save, but you can hit ctrl s (if you're on a pc) to save i think, otherwise go up to "file" and hit "save as..."

for uploading you'll probably want to save it as a .png, but make sure you save it as a .sai file or a .psd file as well in case you need to work on it more.
it's also beneficial to make a folder just for your work.

once you run out of that trial you won't be able to save any work done, and after that point you p much have to buy the license to continue using it. redownloading and reinstalling won't change anything.

if you don't want to do that, i suggest trying out some similar & free programs; https://krita.org/ is a free one, and although i haven't used it, it looks very promising and comes with things that sai doesn't. i might even try this program myself!