Updating CafeOS
New versions are on the Download page; see what changed in the changelog.
The shop server
Run the new server installer on the server PC. It stops the services, updates the program, and starts them again. Your data in C:\ProgramData\CafeOS stays as it is. Update when the shop is quiet: PCs reconnect by themselves within a minute.
The PC clients
Run the new client installer on each PC. It keeps the PC's pairing.