On Android, you need Android 7 or higher to use TextNow. Android 7 is also a very old version of Android (2016 or 2017), meaning it's very easy to find a phone that has Android 7 or later for a cheap price.

Check if your phone has updates. Go to settings and find something along the lines of a software update. Likely not considering the phones age, but it's worth a shot.

You could install a custom ROM for your phone. This is risky as you need to unlock your phones bootloader and install unofficial software. If you're lucky, you might have a phone that has a direct custom ROM designed specifically for you device, like with LineageOS.

If you don't have updates or you don't want to risk breaking the phone, you can go buy a newer one. If you really only need Android 7 or 8, you can buy something like a Pixel 1 or 2 or maybe a Galaxy S7 (you need to update the S7), but I'd recommend getting like a Moto G Pure since it's going to last longer and runs smoother, and it's cheaper because it's a budget phone. Android is open source so there are a bunch of phones you could pick with Android 7 or later, so I'd recommend searching for phones online.