How do I change my Apple ID to the same as my husband's

That will result is all your data being mixed together. Sharing an Apple ID isn't recommended. Consider Family Sharing as an alternative. That said, see the discussion below and Winston Churchill's answer. Use the 2nd link as a guide of where to sign out on your device.

How to transfer all files from one iCloud account to another

What to do after you change your Apple ID or password - Apple Support

Family Sharing

Family Sharing - Use your own Apple ID for Family Sharing

Family Sharing -What types of content can I share?

Family Sharing - If you don‘t see your family‘s shared content

Family Sharing - Ask to Buy

Family Sharing - purchases and payments

Family Sharing - Hide and unhide music, movies, TV shows, audiobooks, and books

Family Sharing – share iCloud Storage

Family Sharing - Share an iCloud storage plan with your family

Family Sharing and location

A better thing for you would be Family Sharing. Sharing Apple ID's can be cumbersome for you and your husband, you'll start getting his messages and contacts and pictures. Family sharing will allow you to share purchased music, locations, pictures and payment methods that you choose to share.

Set up Family Sharing - Apple Support

If you want to use his account instead of your's you can log out via Settings > iTunes & App Store (tap on the id) and log into his instead (I assume that you want to continue to use your own account elsewhere that you're currently using it e.g. FaceTime, Messages, iCloud). If you've downloaded anything from the stores on your account they will remain tied to your account.