How to Modify a Variable when a Request for approval is APPROVED

[Walter Toney]

Good Day, 

how can i change a select box Variable 'hyt_PoC_Signoff' from "Not yet Completed" To "Completed Approved" and Set a Date/Time Field when a Person Approves a request.

my flow is in Workflow and Im using a script and have tried several items and searched for the proper way but not really finding anything that matches my needs. 

this is the code i tried

Var gr = new GlideRecord('sc_req_item');
gr.addQuery('sys_id','current.sys_id');
gr.query();
if(gr.next()){ gr.variables.pool.ffms_poc_signOff = "Completed (Approved)"; gr.update(); }

Any Ideas