Hi SAP Process,
You can not edit a planning area when it is in initialized status (green status). To be able to change it and update the UoM, you need to de-initialize it, make changes and re-initialize.
Please make sure you do not have any time series data in your planning area, else the data loss will be there for those key figures. It's OK otherwise if you are using the std. SNP planning area 9ASNP02.
I think UoM would already be there in your planning area as it is a mandatory field before saving/initializing a planning area, you only should need to change the UoM so that your SNP heuristic can understand it for the SKUs you are running planning for.
Regards,
Umesh