A supplier has become VAT/ GST registered and I need to amend their timesheets

Firstly you should update their Supplier Entry record with the supplier's Sales Tax details and set the tax settings to the correct rate:

This will ensure future timesheets pick up the correct settings.  For any existing timesheets, the system will automatically update timesheets that are Open but for any that are mid-process you will need to update each one separately. 

The best place to do this is from the Timesheet Display option within the Timesheet menu, but you can also do this whilst validating the timesheet if you wish. 

Select the candidate then select the timesheet - the example below shows a rate with both the pay and bill element on one line and if you click [Pay Tax] this pay tax box will drop down for you to amend the amount of Sales Tax you wish to add to the suppliers payment:

If you wish to make any other changes to the pay line please select Show Complex Rates to see the full line 

Finally click Update to save the changes.

If the timesheet has been included on a pay invoice/remittance please recreate the document from within Invoice/Remittance Display.