how to do merge in postgres ("with upsert as" not supported)

hey, guys, I came across a merge statement when I'm trying to convert
stored procedures from Mysql to Postgres:

merge into db.ChargePeriod d

using (

select ba.ClientID

, ba.BillingAccountID

, bs.BillingScheduleID

, @CodeWithholdD as WithholdTypeID

from db.ClientPartyIDConfiguration cpc

join db.BillingAccount ba

on ba.Deleted = 0

and ba.ClientID = cpc.ClientID

and ba.PartyID = cpc.PartyID

and convert(date,getdate()) between ba.EffectiveDate and
ba.ExpireDate

join db.BillingSchedule bs

on bs.Deleted = 0

and bs.ClientID = ba.ClientID

and bs.CoverageBeginDate >= ba.EffectiveDate

and bs.CoverageBeginDate <= ba.ExpireDate

where cpc.Deleted = 0

and cpc.ClientID = @ClientID

) s on d.Deleted = 0

and d.ClientID = s.ClientID

and d.BillingAccountID = s.BillingAccountID

and d.BillingScheduleID = s.BillingScheduleID

when matched

and isNull(d.WithholdTypeID,-1) <> isNull(s.WithholdTypeID,-1)

then update

set WithholdTypeID = s.WithholdTypeID

, UpdateUser = @UpdateUser

, UpdateDate = @UpdateDate

when not matched then insert (

ClientID

, BillingAccountID

, BillingScheduleID

, WithholdTypeID

, CreateUser

, CreateDate

, Deleted

, CancelDate

) values (

s.ClientID

, s.BillingAccountID

, s.BillingScheduleID

, s.WithholdTypeID

, @UpdateUser

, @UpdateDate

, 0

, '9999-12-31'

)

;

I saw some people use "with upsert as", but my pgAdmin version(1.8) doesn't
support it. Anyone has any ideas how to do merge in postgres?

Thanks.