This is a long and complicated discussion aslfm. Which software do you mean?I STRONGLY recommend you print this form and sign it in ink. Anything else may run into legal trouble with the IRS as well as wasting hours or days on the computer.
Notwithstanding this, it is basically discontinued and can no longer be purchased except as part of a suite costing tens of thousands of dollars (as I understand it).You are NOT MEANT to edit these forms, and trying to can cause SERIOUS issues. This often happens with Government forms. Worse, people...
Be warned that this form was designed by IRS so you couldn't edit it. The form you edited might be for your own use, printing out, signing etc, and if so that's great. If it's to send to the IRS electronically, don't do that!
The only product available to you is the one you have, but you should consider migrating. Adobe would say migrate to an Acrobat form. I'd say, migrate to HTML forms. To be clear "migrating" here means "learn a new technology and do the whole job again" not "open in a new app and have it change stuff...
Designer is discontinued, but if you have a license for ES2 that's the only one you can use anyway. ES4 came out many years ago but you can no longer upgrade.