Users looking for sb to sb3 conversion are typically searching for a way to export or convert ancient version 1 projects of Scratch to version 3. It doesn't seem that this is actually possible, because of how the app and its projects evolved and no sb to sb2/sb3 converter exists, despite many users looking for it.
You can upload your old .sb or .sb2 files to Scratch and redownload it as .sb3 file. Some blocks might be undefined and you will have to solve some issues though.