Help upgrading to W10M

Hello! A few days ago I asked if it was convenient to upgrade from WP8.1 to W10M. I have a Lumia 640 LTE with WP 8.1 which, as far as I know, can be upgraded. Now, I tried following a tutorial that explained how to use OTC updater or something like that. When I used it, it would tell me that there were no updates available. Is it an OTC problem? Or is it that the 640 LTE can't be upgraded to W10M?