I am wondering if this is possible at all. I want to update column x if a condition is true, otherwise column y would be updated
UPDATE table SET
(CASE (CONDITION) WHEN TRUE THEN columnx
You can't use a condition to change the structure of your query, just the data involved. You could do this:
update table set columnx = (case when condition then 25 else columnx end), columny = (case when condition then columny else 25 end)
This is semantically the same, but just bear in mind that both columns will always be updated. This probably won't cause you any problems, but if you have a high transactional volume, then this could cause concurrency issues.
The only way to do specifically what you're asking is to use dynamic SQL. This is, however, something I'd encourage you to stay away from. The solution above will almost certainly be sufficient for what you're after.