You didn't say which procedure you are using.  If it is UNIANOVA, you can do this, for appropriate contrasts, on the CONTRAST subcommand.

From the CSR:

SIMPLE. Each level of the factor except the last is compared to the last level. To use a category other than
the last as the omitted reference category, specify its number (which is not necessarily the same as its
value) in parentheses following the keyword SIMPLE. For example:
UNIANOVA Y BY B
/CONTRAST(B)=SIMPLE(1).
Suppose that factor B has three levels with values 2, 4, and 6. The specified contrast compares the other
levels to the first level of B, in which B has the value 2. Simple contrasts are not orthogonal.

If you are using GENLIN, you do this on your BY list with the ORDER keyword:

ORDER Keyword
The ORDER specification following a list of factor variable names determines the sort order of factor
values. This order is relevant for determining a factor's last level, which may be associated with a
redundant parameter in the estimation algorithm.
ASCENDING. Factor variable values are sorted in ascending order, from the lowest value to the highest
value. This is the default order.
DATA. Factor variable values are not sorted. The first value encountered in the data defines the first
category; the last value encountered defines the last category. This option may not be specified if splits
are defined on the SPLIT FILE command.
DESCENDING. Factor variable values are sorted in descending order, from the highest value to the
lowest value.