You should be prompted with two option, app details/info and clear from list tap on app details/info and click on force close this will do your job. you should see viber there at the bottom.

You can learn more about the exciting news over on Viber’s blog. Then open your multitasking window/history window by pressing the home button. Like messages: You can now like messages in group chats.Live Tile: The Live Tile in Windows 10 will show you your new messages.Localization: New language support for both Russian and Portuguese.Organize stickers: Hide and change the display order of your stickers through the Sticker Market settings.Access chats from Start menu: Start a new message or reply in recent chats right from the Start menu.Share to Viber: Share links, text, photos, videos or your location to Viber easily from within any Windows 10 app.Cortana support: Simply ask Cortana with your voice to call or text someone for you.Quick reply: Your messages will pop up outside of the app so that you can reply quickly, without opening up the app, and get on with whatever you’re doing.Video calls on mobile: Now you can video-call anyone from your phone!.Let’s take a closer look at the awesome features in Viber for Windows 10: Share photos, videos and voice messages, message groups of up to 200 people, and if you have second thoughts about a message you’ve sent, just delete it – even after it’s been delivered. Viber is the easy-to-use free app that lets you send messages and make HD-quality phone and video calls securely over WiFi and mobile networks from just about anywhere.

How to use viber in windows on desktop