Can I update iPhone 6 Plus to iOS 15.5.6?

Your iPhone 6 is too old and too limited and is incapable of iOS upgrades past iOS 12, and no hardware upgrades are available, leaving you with the need to replace this iPhone with a newer (used or refurb) or new iPhone, and given the imminent arrival of iOS 16 that replacement also preferably with an iPhone 8 or newer, and any recent iPhone will be substantially more capable than this iPhone.

The "latest" version for the iPhone 6 Plus is iOS 12.5.6. That is as far as it can go regarding updates.

If you want to run a higher iOS, you will need a new phone.