Although there’s no “reset” button, you can easily do it manually without changing or losing any transactions.
On the first of the month, just do a Fill from Unallocated by toggling every envelope to “Set”. (You can even schedule this now if you wish!) This will put exactly your budgeted amount into each envelope. If there are any that you intend to roll over in the future, be sure that future fills change the setting to “Add”. Be sure your account balance(s) are all correct on the first as well, including any transactions that show up in Goodbudget but haven’t cleared the bank, in order to be sure your Unallocated balance is accurate.
If your Unallocated balance is negative after the Fill, it just means you’ve added money you don’t have yet to envelopes. Ideally you would empty some back into Unallocated to be filled later in the month, but if you’re really confident you won’t overdraw you could leave it alone making sure that future deposits go into Unallocated.
Hope that helps!