PS 1.7.x.x: This work for the classic and other Themes: Go to International -> Translation -> In the Modify section select Type of Translation = Theme Translation, in select your theme you select the theme you want change, select the language you want to translate -> click on Modify. A new screen appears. Then in "Search translation" enter Copyright hit on search and there you go. Change it to whatever you want.
However, not all themes support this action. It depends on the Theme you have. Some themes have a Module that lets you change the Copyright.
If your Theme does not support any of the above you can manually change it by signing into your webhosting, going to File Explorer ->public html -> themes -> theme you want to change -> templates -> _partials -> footer.tpl