If there's a typo you are just not setting, then you could also try the other case format:

update mngrnt set rfrnc = case aqno
when '50001' then '155'
when '50002' then '166'
etc
end

> > I tried this
> >
> > update mngrnt
> > set rfrnc=
> > case
> > when aqno = '50001' then 155
> > when aqno = '50002' then 166
> > when aqno = '50003' then 1331
> > end
> >
> > I got SQL error
> >
> > Invalid token.
> > Dynamic SQL Error.
> > SQL error code = -104.
> > Token unknown - line 4, char 1.
> > when.