[plt-scheme] applying set! on the contents of a variable

how does one change a variable whose symbol is stored in another
variable?
 sorry about the beginner question, but i couldn't find an answer in
the group discussions or on google.
 the reason i need this came up was because i have a function inside a
class which takes as input an alist of properties and values and
changes the current values in the class using this alist. i have a
workaround, but was curious as to whether variable names could be
treated as first-class objects.