If you have not billed them then just change the fee arrangement to flat and enter the amount. If appropriate change the bill layout and save. 

If you have already billed them you have a few options

If the bill is undoable, undo the bill and create the flat fee 

If the bill cannot be undone then choose from below options:

1. Give them a credit for the hours billed and enter a flat fee

2.  Enter a flat fee for the total less the amount already billed 

3. Give a credit for already billed. Duplicate all the time already billed and enter the flat fee. If being able to allocate the flat fee across all timekeepers is critical then this might be best. Just remember that timekeepers will have extra hours if you. Look purely at hours worked. 

If you have not billed them then just change the fee arrangement to flat and enter the amount. If appropriate change the bill layout and save. 

If you have already billed them you have a few options

If the bill is undoable, undo the bill and create the flat fee 

If the bill cannot be undone then choose from below options:

1. Give them a credit for the hours billed and enter a flat fee

2.  Enter a flat fee for the total less the amount already billed 

3. Give a credit for already billed. Duplicate all the time already billed and enter the flat fee. If being able to allocate the flat fee across all timekeepers is critical then this might be best. Just remember that timekeepers will have extra hours if you. Look purely at hours worked.