According to the standard design of the application, when you enter a new inventory part with a UoM, it is not allowed to change it.

However, if this is a brand new part which you haven't used in any transaction, then you can simply delete the part and add it again if you really made a mistake in the UoM.

But, please note that if you have used the part in any transaction, then you cannot delete as well

Thank you @Deshani Gunasinghe 

in this case it is a old part and it is in use in othe ojects. This means I can’t change it anymore, right?

@Link answer would be yes ,you can’t change .Only way would be through dropping the inventory part record if it doesn’t involve with transactions 

Thank you also @ShawnBerk