The limits can be found on the left side of the results screen. The middle column lists the results, and the far right colum shows more specific results such as articles that are free in PubMed Central.
PubMed refers to Limits as Filters. You can find them on the left hand side. Choose which options you would like to see from the link at the top of the column.
Once you have selected specific filters, you can turn them on and off by clicking on the word. When the filter is checked, that means it is active. Click again to un-check.