how do i update my itunes to 12.12.5?

In that case it appears that you have the Microsoft Store version which, as stated, is updated automatically. Therefore, you should assume that you do have the latest version installed.

As for Apple's suggestion - to update to the latest software, it's the first thing they always tell everyone to do. However, as you have found, if you have the Microsoft Store version of iTunes, their advice is impractical.

Rather than focus on impractical advice, focus instead on the fact that you cannot get your Home Sharing working: have you read the Apple article Use Home Sharing to share media from a computer to other devices - Apple Support?

The article does begin by mentioning updating iTunes, which I suggest that you ignore since we've covered that issue. However, if you have portable devices, make sure that they are updated too. Pay particular attention to the rest of that section (titled Before you set up Home Sharing).

Follow the steps mentioned in the remainder of the article.

Since it looks as though you have visited the Apple help page about updating iTunes, what is the problem that brings you here?

• If you click on iTunes' top bar menu Help / Check for Updates, you will see if there is an update for your iTunes and you will be able to download it from there. If you check the Help / About iTunes menu option, it will tell you which version you currently have
• If the option to Check for Updates is not shown in the Help menu, that suggests that you have installed the version of iTunes from the Microsoft Store. This version updates automatically, you do not need to do it yourself
• If you are using Windows 7 (or Windows 8), then you cannot install iTunes 12.12.5

You're welcome.