1. Make sure your payscript has 'Weekly' in its WorkingPeriods parameter.

Example:
///WorkingPeriods: Weekly, Semi-monthly, Monthly

2. Create a new paysheet (either blank or next paysheet) based on the paysheet you used in no. 1 with the correct list of WorkingPeriods.

3. While creating the new paysheet, you will be asked if you want to edit the settings. Edit the settings and select your desired working period (Weekly).

Hope this helps.