i see another apple id when i want to update

This situation can happen if there are apps (or media purchases) on the device that were installed (or downloaded) under a different Apple ID, and they need updating or redownloading.

The best way to resolve this is to delete the item(s) requesting the update, and then download it again using the Apple ID that is signed in with the device.

Uninstall apps on your Mac - Apple Support

Once deleted, adding the app using the correct Apple ID should resolve the issue. See the article: Download apps and games on your iPhone or iPad [ Download apps and games on your iPhone or iPad - Apple Support ]

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This is modified from a Community Specialist post: how do i get ride of a old id - Apple Community

he gave me this MacBook

He need to read -> What to do before you sell, give away, or trade in your Mac

What to do before you sell, give away, or trade in your Mac - Apple Support

Then has to remove the iMacBook from his account, see -> Check your Apple ID device list to see where you're signed in -> Check your Apple ID device list to find where you're signed in - Apple Support

Then you can sign in with your own Apple ID.

It is best if you can log in with your own Apple ID.

Apple recommended -> Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

Your brother will never actually give his Password to you.

I am less familiar with doing this on a Mac but simply using his password might not work, it might want you to enter both his ID and password. I know on an iPhone this would have far reaching issues so I would just go with the deleting the app approach.

Note that if his old apps are on the Mac, you may wish to do everything in the link provided in a previous posts, particularly if this is a Mac from the last decade. There would be other things that could reach out and bite you such as not being able to update the system software.

I am thinking about Activation Lock. Click on this link to read about activation lock for Mac--> Activation Lock for Mac - Apple Support