Q: how do I download the 90 free trial

Howdy iaea,

Thank you for visiting Apple Support Communities.

Apple Music free trial is not something you download per se, but you sign up for it through iTunes.

Join Apple Music on your iPhone, iPad, iPod touch, Mac, or PC - Apple Support

Join on your Mac or PC

1. Open iTunes. Make sure that you have the latest version.
2. In the upper-left corner, click the Music  icon.
3. From the top of iTunes, click For You.
4. Click Start 3-Month Free Trial.
5. Select Individual or Family Membership, then click Next.
6. If prompted, enter your Apple ID and password that you use with the iTunes Store. If you don't have an Apple ID, click Create Account.
7. You might be asked to validate your billing info, but you won't be billed until after the three-month free trial period ends. Add a valid payment method* and click Join.
8. If prompted, agree to the terms and conditions.
9. Choose genres and artists that you like.

You may also want to check out, About Apple Music - Apple Support

Enjoy.