Du kan dela upp XML till en tabellvariabel, ersätta varje nod separat och sedan kombinera dem igen.
declare @xml xml =
'<a abb="122">
<b></b>
</a>
<a abb="344">
<b></b>
</a>'
declare @T table (XMLCol xml)
insert into @T
select
a.query('.')
from @xml.nodes('a') a(a)
update @T set
XMLCol.modify('replace value of (/a/@abb)[1] with 888')
set @xml = (select XMLCol as [*]
from @T
for xml path(''))