in RUN u can give services.msc
here u will find number of db2 services. The important one of these is
db2 -db2 -0. right click this service and in properties of this, there is a logon tab. here u will find the information u need to make changes too.
For ur knowledge, also check the dependencies in the propeties of a particular sevice. if any dependency exist for a particular service, then plz make changes\restart the dependent service also.