Activate new Wordpress theme to replace an outdated one

I am trying to update to the latest php version on my Wordpress site, but an outdated theme is incompatible and is causing a critical error. I need help to activate a new theme that gives the same look to my site and which is compatible with the latest php version. The job will involve getting the slider and all other functionality back up and running and to mirror the same look as before with the old theme.