my iPhone 6 is charging for an hour and still showing the sign of low battery in red

Potentially, it could mean that this iPhone's battery has passed it useful life. Is the battery the original one that came with this phone?

You may find the following support article helpful: If your iPhone, iPad, or iPod touch won't charge - Apple Support

Apple considers that an iPhone battery needs replacement if it cannot maintain, at least, an 80% charge after 500 full recharging cycles. If this is the case for your iPhone, then you should consider getting it replaced by Apple or an Apple Authorized Service Provider.

Ref: iPhone Battery & Power Repair - Apple Support