buy and install the SSD, install windows 11 iso into the usb, put the usb in the laptop, when it prompts to install windows 11, then install it.
windows 11 ISO. select"Create Windows 11 Installation Media
If you want to perform a reinstall or clean install of Windows 11 on a new or used PC, use this option to download the media creation tool to make a bootable USB or DVD."
https://www.microsoft.com/software-download/windows11
you will NOT risk losing any data since you are buying new SSD and putting new one in. if somehow you can not install it, then you can always install the old one back.
once the new one is installed and windows 11 is ready, then you can buy an external SSD case for like $10-20, then slowly move your data from the old HD to new SSD.
again, not hard to do, but time consuming.