You can use Float's Spreadsheet Input tool to easily copy and paste your forecast from elsewhere into Float. This lives on the Cash Flow tab above the graph.

While you can't upload directly from a spreadsheet, you can certainly copy your budgets right in.

Set a budget for any date within a month by using the date selector:
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Make sure the numbers are formatted without any symbols or commas, then use ctrl + C and ctrl + V to paste rows from your spreadsheet in and hit save.

This can be repeated for budgets set for a different date.

You can also use Spreadsheet Input to move easily between cells with your keyboard arrow keys, like you would in a spreadsheet, and enter your budgets more quickly. And you can click and drag a cell to the right to copy it forward or down.

Spreadsheet Input is currently available in the 'Base' scenario but not on scenario layers