Top right of screen, click on the little cog, go to account info, and there is a link under Business Information to transfer account.

Be very careful though, as if you sell to someone who already has an account, or has had an account in the past they could potentially get suspended for having a linked account, especially if they have ever been involved in an account that got suspended or had problems, whilst you can have 2 accounts these days, you are not able to sell in the same category across 2 accounts.

You will also need to make sure that all the details on companies house are fully updated, and reflected within seller central, make sure you remove all your personal information, and especially any credit cards etc.

Also be aware that even when you transfer the account, you will still be linked to that accounts history, so it could cause you an issue if you wanted to open a new account in the future, this can normally be resolved by proving to Amazon that you sold the account, but you should be aware that they can still link the account even after you come of it, meaning you would have to go through this process.