To change a project's budget, visit the project's page.
Look in the white box at the top of the page, next to the section entitled "Budget." You should see a yellow pencil icon next to the current dollar amount. Click that icon to increase the budget. You cannot decrease the budget after a freelancer has been assigned to a project.
(These instructions apply to project-based projects. You cannot change the hourly rate for an hourly project after a freelancer has been assigned to it.)