When compared to its predecessor, CKEditor 5 should be considered a totally new editor. Every single aspect of it was redesigned — from installation, to integration, to features, to its data model, and finally to its API. Therefore, moving applications using a previous CKEditor version to version 5 cannot be simply called an "upgrade". It is something bigger, so the "migration" term fits better.
There is no "drop in" solution for migrating. Please refer to this guide to see a summary of the most important aspects to be taken into consideration before you proceed with migrating to CKEditor 5.