DIAGNOSIS The chuck isn't stuck, you're missing the release step. The SDS-plus chuck on this model is held by a retaining clip, not threaded on.
FIX Pull the chuck forward away from the tool. You'll see a black, horseshoe-shaped retaining clip in a groove. Use a small flathead screwdriver or punch to pry that clip out of its groove. Once the clip is removed, the entire chuck assembly slides straight off the tool shaft.
IF THAT DIDN'T WORK Concrete dust and debris can jam the chuck on the splines. After removing the clip, tap the chuck collar gently with a rubber mallet to break it free. Do not hammer on the tool itself.
HEADS UP When reinstalling, ensure the new chuck slides all the way on until the groove is exposed. Then install the retaining clip fully. A missing or loose clip is a major safety hazard.