"To many rows being processed at one. Reduce and try again." error in SQL Server Management Studio
To clarify on @McNets comment: The error isn't produced by SQL Server itself. Somebody created a trigger on the table, a trigger that fires on UPDATE. This trigger apparently does a check for number of rows and if above a certain threshold, it generates this error and does a rollback.
You should try to find who is responsible for the trigger and take it from there. Perhaps it is OK to do an exception to that trigger rule in this particular case? Etc.