As the error message says, there are two options.
First, you as a developer should modify the version number of the ClickOnce manifest. This is done at the ClickOnce tab of the project settings. Note that ClickOnce version has nothing in common with your assembly (assemblies) version.
Another option is to uninstall the application and unfortunately, this probably has to be done manually. You have to find the ClickOnce deployment folder (C:\Users\username\AppData\Local\Apps\2.0) and manually delete all folder contents. This can also be done by invoking the
rundll32 dfshim CleanOnlineAppCache
command from the commandline.