Groz,
For Firefox: right click on the menu icon, it is three parallel lines and It's all the way to the right on the top.
A window opens and you will see an options icon, it looks like a gear, right click on it.
A new tab will open up and you will see a list on the left and various options on the right. By default, you see the options for general, right click on privacy.
In the middle of the page, you will see remove individual cookies in blue, right click on the words.
A window will pop up that says cookies.In the search bar type app.com then hit enter or click on the magnifying glass. You will see a list of cookies, right click on remove all, then right click on close
Then you can close the tab.
The other common browsers are similar, I'll post similar instructions for IE and Chrome
Feel free to reach out if you have any issues