How can I update iOS 14.4.2 to 15.8.1

You can't update the phone until you follow the directions given: you need at least 3.57 GB of free storage. You will also need to connect your phone to WiFi.

I would recommend you update your phone by connecting it to a computer, backing it up and restoring it from the back up. That may help with your memory issues and updating from a computer requires less free space on the phone.

I'm inclined to agree with tonysit. Your phone is in desperate need of a new battery. Batteries should be replaced once they drop below 80%. Frankly, because of the small capacity, I'm not sure it's worth spending much money on this phone. But, if your only option is to replace the battery, start here:

iPhone Battery Repair & Replacement - Apple Support

This may help --> Manage and Clear Storage on your iPhone a… - Apple Community

16GBs of storage, Battery needs replacement and this device cant update pass ios 15. Everything here says; it really is time to consider a new phone.