Recently I saw one of my connections on here, 
This gave me the idea to create a quick guide on how to accomplish this.
I will go over how to do this on two different platforms:
- InLeo
- Peakd
When should I do this
If you suspect there is an unauthorized person who has access to your account, you should perform these steps ASAP
InLeo
Contains direct links, clickable. Opens in a new tab
In your settings page, scroll all the way down and click on CHANGE KEYS
Here you need to enter your Private Owner Key and also set a new Master Password
When this is done, click on "Download Keys".
I'm not doing this for the article here, but after you've downloaded them and they keys have been changed, you will need to either update your keys in your Hive Keychain or re-import your account.
Peakd
From the profile menu, go to one of the menu items shown here
When the page has loaded click on account actions and then go to Keys & Permissions
After this you go to "Change Password"
You need to confirm you have read the text before you can update your keys.
Note: Peakd does not let you set your own Master Password
Enter your Owner Private Key and click generate new password.
I'm not doing this for the article here, but after you've downloaded them and they keys have been changed, you will need to either update your keys in your Hive Keychain or re-import your account, as it also says on the page you do this from.
Final words
I hope this short guide has helped you out a bit.
If you have any questions, get in the comments and ask away.
Images are screenshots from InLeo and Peakd, respectively. All captured by me while writing the article.