The first thing you should do is determine how large the Users files are on your SSD. These are the "new" data files you have added and saved to the SSD while using Windows. These files are irrelevant to upgrading to Windows 10. The space these files take up could be space you need to "upgrade" to Windows 10 directly. You can move these files temporarily to a USB flash drive, then delete the files from the boot drive (the "C" drive).
If emptying your "Users" folder doesn't free up enough space to install Windows 10, you need a larger SSD. Sorry.
SSDs are much cheaper now than they used to be. Splurge.
Removing and replacing Users Folder files:
Open a file window and navigate to This PC > Windows, then select the C drive --which is...