Gutenberg is the new editor for wordpress which offers blocks to write the texts, upload images etc.
If you have an older posts which has been written with old Classic editor and now if you want to update those older posts, its better to switch to new Gutenberg editor and make the post contents compatible with new editor.
WordPress provides a ready option which can automatically convert your old posts to new Block editor. The option is available as below,
Above image shows, how we converted our on of old post written using Classic editor to Blocks. For doing this, click on “Top Right” cornered 3 dots and then click “Convert to Blocks” and wordpress will convert your old post to block editor compatible.
That’s it.. but please review manually once if anything has misplaced in your old contents during this auto conversion before publishing this post again.