i bought baldi basics plus, but only 0.1.5 is avalivable. can someone tell me how to download older vesion of games?

If you are browsing Itch.io from the web, your only option is if the developer has made an older version available. If you are using the Itch app, and a developer is using the tool butler to upload new versions of a game, then older versions will be automatically available. This should be available in the download screen, there should be an option for downloading an older version.