Then you changed it before version 1.8.2 so there is no need to remove anything.
If you see all the needed php versions then it is installed.
v-lists-sys-php
To change the php version for your site you need to edit the web domain and select the right php version on Backend Template PHP-FPM field.
As you didn’t talk about Apache, I suppose you are using Nginx standalone.
When Nginx doesn’t start:
systemctl restart nginx
systemctl status nginx --no-pager -l
You should also check (replace YourDomain by the actual domain):
/var/log/nginx/error.log
/var/log/nginx/domains/YourDomain.error.log