Hello there, Rabi_00.
It sounds like since updating your iPhone 5s to iOS 8.4 you feel it is running hot and may be broken. Depending on the warmth of the iPhone, this can be normal behavior. The following Knowledge Base article provides some great information in regards to just that:
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support
When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.
Your iPhone will warn you if it is over heating, and if it needs to, it will shut itself down. As long as you have not experienced this warning, what you are sensing may be normal. If you believe the temperature is a concern, you can isolate the issue by restoring your iPhone as new. This will help you identify whether it is a hardware or software issue:
Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support
Thanks for reaching out to Apple Support Communities.
Cheers.