No you can't upgrade a a Nokia Lumia 710 to WP 8.1. You can only update it to the maximum of WP7.8. If you want WP8.1 you gonna need a new phone.
I am a past user of Lumia 710 myself it's a great phone, but I've bought a new Lumia 1320 on 2014 to get to WP 8.1. It is much better now, specially that I am an insider of Windows 10 mobile too. It's very worth i have got it.