Recordset.Edit or Update sql vba statement fastest way to update?

Assuming WHERE columnc = 20 selects 1000+ rows, as you mentioned in a comment, executing that UPDATE statement should be noticeably faster than looping through a recordset and updating its rows one at a time.

The latter strategy is a RBAR (Row By Agonizing Row) approach. The first strategy, executing a single (valid) UPDATE, is a "set-based" approach. In general, set-based trumps RBAR with respect to performance.

However your 2 examples raise other issues. My first suggestion would be to use DAO instead of ADO to execute your UPDATE:

CurrentDb.Execute stSQL, dbFailonError

Whichever of those strategies you choose, make sure columnc is indexed.

The SQL method is usually the fastest for bulk updates, but syntax is often clumsy.

The VBA method, however, has the distinct advantages, that code is cleaner, and the recordset can be used before or after the update/edit without requering the data. This can make a huge difference if you have to do long-winded calculations between updates. Also, the recordset can be passed ByRef to supporting functions or further processing.