To move to a different field, call sfm_nextitem() specifying the actual number of the item that you want to go to or an appropriate action code to go to another item on the form.
If you wanted to move to the previous item, you could specify curitemnum(dfmp) - 1. Alternatively, you could specify AC_PREVITEM, one of the action codes for moving between items on the form.
If you do not know the number of the item you want to go to, you can obtain its number from one of the functions that translates item identifiers. For instance, if you know the name of the field, you can obtain its item number with i_namnum().
You should ensure that the item number that you specify is within the range of item numbers for the form. The number of the last item on the form is i_maxnum(dfmp).
