First off, there are no “false numbers” from previous incomes. If your account balances are accurate and you have properly entered all income and expense transactions, then you have true numbers. They may not be the numbers you want in that moment, but they are true.
So first, are your account balances accurate? Have you recorded all your expenses to date? Are you entering Income when you receive it? Are you allocating it at that moment to envelopes or leaving it unallocated?
At the start of a new budget period, technically all your money is “leftover” from the previous month(s). Everything allocated plus anything unallocated (i.e. the sum of your account balances) is what you have available to budget with. If you want to also include the future income from the upcoming month in your budget then that’s a reason to create an additional account, and I have detailed how I do that in a previous thread: My savings are unallocated? - #8 by kschedler
Generally it’s a bad idea to clear your data and start over unless you have a really compelling reason to do so. It makes your life harder and you lose the ability to report on previous months. It’s actually a good thing to have leftover funds from last month, it means you are living within your means and building up a safety net. The only question is how you want to record it.
As detailed in that other thread, at the end of the month I move all the “leftovers” into a goal envelope named “Slush”. This leaves the monthly envelopes a “clean slate” for filling from the next month’s income. GB is designed not to count future income, though, so the “normal” way of using it is only allocating what you actually have in your accounts at the start of the month. The way I use it (and it sounds like you want to do something similar) requires several workarounds.