To change the bot's name from Nikabot you just need to follow these steps:
1 - In Slack, click on Nikabot's name;
2 - Click on Settings so you are directed to the settings page;
3 - Scroll down to the bottom of that page where you'll find a section called "Bot User" right above the one that says "Remove Application";
4 - There you can click on the pencil like icon and change the name to the one you desire.
The name you give to Nikabot will appear on your Slack under apps, but it will not change the name that appears when the bot prompts you. That one will remain Nikabot, Slack only allows you to change the one under your apps list.