One important step is missing in all answers. I successfully upgraded with following steps:
- stop apache service with wamp stack manager.
- rename your wampstack/php dir to wampstack/php_old
- copy new php dir to wampstack/
- replace wampstack/php/php.ini by wampstack/php_old/php.ini
- test and fix any error with php -v (for example missing extensions)
- [optional] update php version in wampstack/properties.ini
- Replace wampstack/apache/bin/php7ts.dll by wampstack/php/php7ts.dll
- This is not mentioned in the other answers but you need this to use
the right php version in apache!
- start apache service