The very short answer is that - out of the box - you won't find .htaccess in the Kentico admin
Kentico runs on Microsoft Internet Information Services (IIS) and does not support .htaccess files. You can however install an extension to IIS to support URL rewriting. You can find the Url Rewrite extension here. The Url Rewrite extension can help you to import any existing rules from and .htaccess file in to IIS. The process is fairly straight forward and is documented here: Using the .htaccess rules in web.config. These rules are now in your web.config file and you can maintain them going forward.
If you want to be able to manage the redircts from within Kentico, there are a couple of modules options.
Firstly, there is an article by Jan Hermann on devnet about URL Redirection that links of to a webinar recording and also a package that you can download (obviously, make sure you read the article first).
Secondly, on the market place there are a couple of modules that may be worth looking into if they're maintained on your version of kentico: