If a previous version is still running in the background - a hanging application, endless loop, hidden window ended with END and not SYSTEM (countless possible reasons) - then QB64 won't be able to overwrite the older file. Before you hit F5, please check Task Manager to see if that's the case.
If it's not, please check that you have full write permissions to your QB64 folder.
If that's settled, check that your antivirus/Anti-Malware isn't grabbing hold of your binaries for verification everytime you run them - it's always wise to whitelist your whole QB64 folder in those types of software for smoother operation.