This is a two step process. First you upgrade the server (or cluster) and then you need to upgrade every database.

To Update the Server

To update PostGIS on the server run the installer found here: http://download.osgeo.org/postgis/windows/pg90/postgis-pg90-setup-1.5.5-1.exe

To Update the Database

n.b. this needs to be run for every database where PostGIS has been installed

First connect to the database and run following the SQL

select postgis_full_version();

you should get 

POSTGIS="1.5.5" GEOS="3.3.5-CAPI-1.7.5" PROJ="Rel. 4.6.1, 21 August 2008" LIBXML="2.7.8" USE_STATS (procs from 1.5 r7360 need upgrade)

Connect to the database as the postgres user and run the scripts C:\Program Files (x86)\PostgreSQL\9.0\share\contrib\postgis-1.5\postgis_upgrade_15_minor.sql.

You can run this in pg_admin but remember to connect as postgres i.e. not iShareData.

Now when you run the SQL statement

select postgis_full_version():

You should  get 

POSTGIS="1.5.5" GEOS="3.3.5-CAPI-1.7.5" PROJ="Rel. 4.6.1, 21 August 2008" LIBXML="2.7.8" USE_STATS