I think your query needs to look something like this:
UPDATE SII SET SII.brandId = @brandId
FROM stringInstrumentItem SII JOIN brand ON SII.brandId = brand.brandId
WHERE brand.name = @oldBrandName
But that would make no sense, as you already JOIN on the same brandId, so maybe this is what you need:
UPDATE SII SET SII.brandId = @brandId
FROM stringInstrumentItem SII, brand
WHERE brand.name = @oldBrandName