SQL update one table select values from two

["Art Tostaine, Jr." <atostaine@xxxxxxxxx>]

I need to update one table based on values from two tables.

This is the select that works, but I want to update values in whssd00.

select count(*) from whssd00
join ptist00 on sddid# = stdid#
where (sdsps# = '131357867' or sdsps# = '131357868')
and stshtocity = 'RENO'

How can I change this SELECT to update the value in SDSPS#?

Thank you.