How to perform the risk assessment in ISO 9001:2015 ?
The standard does not require full scale risk assessment. All it requires is to identify risks and opportunities and this can be done by organizing brainstorming session with relevant people in the company and discuss risks and opportunities, or you can u se SWOT or PEST analysis, or if you like to implement the risks assessment, you can apply FMEA or some similar methodology. For more information, see: How to address risks and opportunities in ISO 9001 https://advisera.com/9001academy/blog/2016/06/21/how-to-address-risks-and-opportunities-in-iso-9001/
And the companies like us who have integrated management system , how can we incorporate these changes in current system?
It depends on the standards you've integrated. If you implemented ISO 9001 and ISO 14001, there are a lot f similar changes and once you make updates for ISO 9001 you can make them for ISO 14001 as well. But the basic principle is to conduct a GAP analysis and determine tho what extent your current system is compliant with new version of the standard and what needs to be changed to achieve the full compliance. Then you can stat updating your management system. For more information, see: How to make the transition from ISO 9001:2008 revision to the 2015 revision https://advisera.com/9001academy/blog/2015/10/06/how-to-make-the-transition-from-iso-90012008-revision-to-the-2015-revision/