Is your AppMon version 7.0, because collectors need to have the same or lower version (eg. 6.5, 7.0, ..) than the Server.
If your AppMon version is 7.0 then you will have to install a new collector with the same name you have in the previous version then stop the service of existing collector and use the migration tool to migrate the data from 6.5 to 7.0 collector(s).
If the above is not required in your case then simply install the new collector 7.0 and uninstall the old one.
Review the below documentation for step by step migration process.