You cannot "upgrade" to Premiere Elements 15 which is the video editing program. That version is no longer sold by Adobe, although you may find it available on eBay or other online seller. But be careful as there are many illegitimate sellers out there.
The current version of the program, Premiere Elements 2020, is now being sold for $99. But other retailers such as Amazon or Best Buy (in the US) may have a better price. Adobe is selling both Photoshop Elements and Premiere Elements 2020 for $149.
If you only want to buy Premiere Elements 2020 and continue using Photoshop Element 15 you should be aware of the following:
You cannot use the same Organizer catalog for each version. Elements Organizer comes with both programs but it will only work with the version that it came with.
You will not be able to use your Elements Organizer 15 catalog to send your media to Premiere Elements 2020 via a menu command. (But you can click and drag media beween the programs, using Windows.)
Video exported in PRE 2020 will not be automatically saved in the Elements Organizer 15. But if you save the video to a watched folder, the EO 15 Organizer will pick it up.