Merge with more complex condition

Hi all, we doing migration from MSSQL to BigQuery.
All our ETL processes convert to DBT. Now I meet a some issue how can I apply MERGE when some of conditions looks like:
[

when matched and [col a] <> [col b]
then update [col a] = [col b]

]
I need to update rows only when matched and only when its not equals

Thanks