You can easily edit labels for these widgets in the following way:
1. Navigate to Administration =>Labels & Messages
2. Look for Resource keys: RememberMe, LogInText, LogOutText
3. Click on the label and edit it with your custom text.
4. Make sure you restart your app afterwards for it to take effect.
Here a link for more detailed documentation which you can find also useful:
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items