WotC has published a handy guide to upgrading your D&D campaign to the 2024 rules

pcgamer.com:

Both the Player's Handbook and Dungeon Master's Guide for D&D 5th edition's 2024 update are now available, and backwards-compatible with the rules published all the way back in 2014. (Back when the world was young and I was first running Lost Mine of Phandelver.) To help players make the change, Wizards of the Coast has published a guide to updating your campaign to the new rules.

Some previous rules changes have been accompanied by multiverse-shaking adventures to explain things like magic working in a totally differently way, like the Die Vecna Die! module that accompanied the shift from AD&D 2nd edition to D&D 3.0. This is a much less seismic shift. Instead, we get some common sense advice on making changes piecemeal rather than all at once, and a set of bullet points on rule changes of note.

A couple of those address whole new systems for crafting and weapon mastery, but others provide an invaluable list of things you might not even have noticed were different. For instance: the stunned condition no longer drops your speed to zero, presumably to differentiate it from the extremely similar paralyzed condition. Being stunned still breaks concentration, takes away your action, gives attacks against you advantage, and makes you auto-fail Strength and Dexterity saves, but you can still take your normal move. Now it's more like when you get hit in the head so hard your world is rocked and you stumble around in a daze, which is a useful distinction, and since you lose your action it's not like you can dash away.

Grappling and shoving are now initiated with an unarmed strike, where before they required an Athletics check. By my reading that's a notable change because it means you can trigger them on an attack of opportunity, where before that was impossible by rules-as-written. Someone tries to run away from you, now you've got a chance to grab 'em not stab 'em.

Healing magic is more potent, with spells that give you back hit points now doubling the number of dice