The very first thing here is to make a bootable OS installer USB drive. Follow this guide: [https://www.macworld.com/article/3092900/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive] as it will make the job easier. But we have a small problem Apples certificates expired so you’ll need a new image file get it here [https://support.apple.com/en-us/HT208202|How to upgrade to macOS Sierra] jump down to Step 4. Here’s a bit more on this [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|f you've got an old macOS install image, it will probably stop working today]
Before you alter things you do what to break the fusion drive set [https://www.lifewire.com/split-fusion-drive-apart-2260166|Split Your Fusion Drive Apart] as the systems firmware needs to understand you don’t want to create a Fusion Drive afterwards.
Once you have that done you can then take both drives out following [guide|11716]. You also have a second drive (SSD) which is just below it following this guide from Step 19 [guide|11713] so you can take the second drive drive out.
Mount your replacement drives and then with your USB plugged in you should be able to see and format both drives to GUID Journals file system (HFS+)
OK, But, I want a newer version of macOS. Why are you having me start here. Often times I find this works better as your systems firmware needs to be updated to support APFS file system. I don’t recommend going to High Sierra, instead go directly to Mojave from the OS updater [https://support.apple.com/en-us/HT210190|How to upgrade to macOS Mojave]