To get started open a revision and select 'Edit' from the action bar, from there, navigate to Review Data:
From the dataset preview, select a column you are looking to set as a row identifier by opening the menu dropdown and clicking the option.
NOTE: Date column types cannot be used as a row identifier.
Once that is complete an ID icon will appear next to the column data type:
Finally, update or publish the draft and the dataset will now have a row identifier!