I bill this position weekly, not daily

Click the unit label above the middle number on the Cost Item and choose Week. That line is then priced per week, and the other lines in the budget stay in days or in whatever unit they use.

Each Cost Item carries its own unit, so a weekly editor and a daily runner sit side by side in one budget, each priced the way it is billed.

Change the unit on one line

Click the unit label above the middle number field on the Cost Item. Until you change it, it reads Days, or Shoot on a line where prep and wrap days are switched on.
The picker opens with the search box ready. Type into Search Unit. It matches on the unit's name, its short form and its category, so "wk" and "time" both find Week.
Click Week. The unit is saved straight away.
Put the weekly rate in Fee and the number of weeks in the middle field. Amount still multiplies, so two editors for six weeks is 2 in Amount and 6 in the unit field.

The same picker covers other billing units, such as Month for an edit suite, Hour or Each.

Switch on the units your company bills in

The picker lists the units your workspace has switched on, and the Configure row at the foot of the picker opens Settings at Units. There, each unit is a chip you click to switch on, and the change saves as you click. The panel describes it: Decide which Units you would like to use in your Budgets. Changes apply to your entire workspace.

Click Configure at the foot of the picker.
Use the Category filter, or type in Search Unit..., to find the time units.
Click the chip for Week, or for Month or any other unit your team bills in.
Return to the Cost Item. Week is now in the picker.

Units are set once for the whole workspace and managed by your Admins, so a team that bills weekly sets it up once and every budget has it. See who should be an Editor and who an Admin.

Good to know

A unit you switch on is available in every budget in the workspace, so a weekly rate works the same way on your next job.
A line keeps showing its unit's name, so a budget that mixes days and weeks reads clearly line by line.
Combine units with the rest of the line as usual: Fee times the number in the unit field times Amount.