Box API-How to Regenerate Access Token from Refresh Token

SK_BoxAPIUtility.apxc

public class SK_BoxAPIUtility {   
    //static method to get access token from refresh token
    public static List<string> regenerateAccessToken(string clientId, string clientSecret, string refresh_token){
		list<string> tokenlist = new List<string>();
		HttpRequest req = new HttpRequest();
		string endpointUrl='https://api.box.com/oauth2/token';
		req.setbody('grant_type=refresh_token'+
			   '&refresh_token='+refresh_token+
			   '&client_id='+clientId+
			   '&client_secret='+clientSecret);
		req.setMethod('GET');
		req.setEndpoint(endpointUrl);
		req.setTimeout(120000);
		//Send request to Box
		HTTPResponse res=performCallout(req);
		System.debug('*response body:'+res.getBody());
		string Response=res.getBody();
		System.debug(Response);
		// Parse JSON response to get refresh_token values.
		JSONParser parser = JSON.createParser(Response);
		while (parser.nextToken() != null) {
			if ((parser.getCurrentToken() == JSONToken.FIELD_NAME)){
				String fieldName = parser.getText();
				parser.nextToken();
				if(fieldName == 'access_token') {
				  string newaccessToken=parser.getText();
				  tokenlist.add(newaccessToken);
				}if(fieldName == 'refresh_token') {
				  string newrefreshToken=parser.getText();
				  tokenlist.add(newrefreshToken);
				}
			}
		}
        system.debug('*****tokenlist:'+tokenlist);
        return tokenlist;
    }
    public static HttpResponse performCallout(Httprequest req){
        Http h1 = new Http();
        HttpResponse resp = new HttpResponse();
        if(!Test.isRunningTest()){
            resp= h1.send(req);
        }
        return resp;
    }
}