Hi Khalid,
Take a look at https://atlas.mendix.com/index3.html#/upgradetheme/7881299347898769
for upgrading your application. Otherwise start a new project based on Atlas and import your modules.
According to Arjan Hendriksen in this topic unfortunately it's not possible. Web Modeler only works for projects newly created using the Atlas framework. Upgrading your theme will not enable the web modeler.