I want to start over with friends.
But all of them already played it a few years ago.
So when they connect via Facebook they only can continue their games.
Is there a way to reset Accounts to start from Level 1 together?

No there isn't unless they start new with a different facebook account.

Facebook account is bound to only one Monster Legends account. So that means, your friends should make new fb accounts to start fresh games.

Alright and how can you change to your new profile in game?

@sebastián-bick First, he must log with his new account in the Facebook app. In the game settings there is an option to change player. And you get two options, one of them is Facebook. You press it, then it automatically tries to log in the new Facebook account.

Warning: You may lose your account in the future because facebook, at random, disables "fake" accounts. So if you and your friends have your "true" facebooks locked onto your current accounts, and you create new "fake" facebook accounts, be forewarned that facebook may disable your new accounts and you won't be able to sync your game to any other device in the future. Any current games you have synced will remain active, so you'll be stuck with whatever devices you have now forevermore.

The other option is googleplay. This can also be tricky with a new device. If you buy a new phone, you'll need to swap over all apps from old phone before it lets you grab your account. If you otherwise just download ML without doing this, you'll get an error and won't be able to grab your old account.

@monsterbath You can add a second Google account to your device and link ML to that account. However, the only way this works is by deleting ML from your device, then log into your second Google account from a computer, go to Google Play, find ML and then send it to your device for install. If you try to install the game on your device directly, even if you switch to your new account in the Google Play app, it will always download and link it to your main account!!! This also means that your in-game purchases go through your second account, so you need to add a credit card or redeem a Google Play gift card @Sebastián-Bick