Here is a step-by-step guide on how to add an SSD as a boot drive in Windows 10:

1. Backup your data: Before making any changes to your system, it’s important to backup your data to prevent any data loss.
2. Purchase an SSD and a SATA to USB cable: You’ll need an SSD that is compatible with your system, and a SATA to USB cable to connect the SSD to your PC.
3. Connect the SSD to your PC: Connect the SATA to USB cable to the SSD and the USB port on your PC. Your PC should recognize the SSD as an external drive.
4. Clone your current boot drive: You can use software such as EaseUS Todo Backup, Macrium Reflect, or Acronis True Image to clone your current boot drive to the SSD. This will create an exact copy of your current drive, including the operating system, files, and applications.
5. Disconnect your current boot drive: Once the cloning process is complete, disconnect the current boot drive from your PC.
6. Connect the SSD as the primary boot drive: Connect the SSD to the SATA port on your motherboard, and make sure that the SATA port is set to the first priority in the BIOS.
7. Boot from the SSD: Restart your PC, and make sure that it boots from the SSD by checking the boot priority in the BIOS.
8. Verify that Windows is running on the SSD: Once your PC has booted from the SSD, you can verify that Windows is running on the SSD by checking the drive properties in the Disk Management tool.
9. Optimize the SSD: You can optimize the SSD for better performance by making changes to the default settings, such as enabling TRIM, disabling defragmentation, and disabling hibernation.