I just don't understand why you would even do that if you've got a stable and functioning engine running at the exact same release number. Well, how did you get it to 1.6.1 in the first place? If all else fails you could do a fresh install and do that again. Just be aware that a fresh reinstall will mean you need to re-add anything you've done to change the base files, like if you added resources to the generator you will have to add those back in (or save the folder and copy it back in when you're done). You followed the directions and backed up your projects first, right? Are you sure you upgraded to this version of 1.6.1? The reason why it says stable in the title is that they had to temporarily downgrade for a while because the first release of 1.6 was buggy, due to no one reporting the bugs in the pre-release version. There was a big slow down in that version, so if you somehow upgraded to the wrong one, that might be why. I don't think it should be available anymore, but I wanted to point it out just in case. You can also just downgrade like it says in the instructions and see if that fixes things.
For the future, just an FYI. It's not always a good idea to upgrade an engine in game dev. Yeah, it's got all the latest bug fixes and features and that's what all the new plugins are using, but it can also break your game or your plugins. If it works and you don't need the stuff in the upgrade, you shouldn't mess with it until you've completed the game.
How to do about ( you can revert to older versions ) ?
the easiest way is by using steam (the key for the standalone version works as a steam key as well, no need to purchase it on steam). If you have the steam version active, then you can go into the beta tab of the steam program properties of MV and select an older version. After that, update your project to that older version in the same way you update it to a newer version.
If you had used the standalone version then you would have to look for the installer of the older version, deinstall the current version and install the older one.
that is probaby the reason - the small updates only work on exactly the previous version, they cannot skip versions.The version I've been using is 1.5
You're in luck, there's actually an entire thread about it: https://forums.rpgmakerweb.com/index.php?threads/change-to-people2-sprite.96927/