It's hard to tell from the images, could you upload a workbook?
Without seeing more, one option you could do if it is the dashboard is on the worksheet, hide the header and float a text field (with tweaking) over it.
Aliases currently cannot be defined for continuous dimensions and date fields. See Aliases in the Product Help. You can vote on the idea to add this as a feature here: https://community.tableau.com/ideas/2295
As a workaround, try changing the date to Exact Date & Discrete. Then format the date as needed.
If you are using a Tableau Server data source, you cannot change the aliases for the fields defined in the data source. The aliases are considered part of the intrinsic configuration for the field. You can only change aliases on calculated fields that you create in your workbook (i.e. that did not come from the data source itself).
If the data source is not one that you published, your only option is to create a duplicate of the field. You can edit the aliases on the duplicate without restriction.
If the data source is one that you published, you can always republish the data source with your aliases configured. Then, any workbook using the published data source will use the aliases that you published.
If your data source is not Tableau Server… then none of the above applies and I apologize.