There are two versions of Minecraft for PC, the Java and Windows 10 Editions. Both editions can be downloaded from the Official Minecraft website. When you click on ‘get game’ the next page will ask you what OS your using. For Windows PCs you will see both Java and Windows 10. The two editions are not compatible and have a few differences. If you wish to cross-play with any other consoles or platforms you will need Windows 10 Edition. Make sure you look into each version to determine which version is right for your personal needs.
If you would like to change the version purchased, you must contact the correct support team to request a refund and exchange your copy for the correct one that you want. Read more about this process here.