There is already a button on the Access toolbars to do this, but if you want a button on you form then you first need to create a macro.
In the Macro design on the first line select Run Command in the Action column.
In the lower pane select the ExportExcel option in the Command field.
Save and close the macro, give it a name.
Goto to your form design, click on the button option and insert a button using the Command Button Wizard.
Select Miscellaneous then the Run Macro action and click Next.
Select the macro you just created form the list displayed.
Click Next and give your button tex or icon, click Next.
Give your button a name and click Finish.