To edit /Login.aspx you need to open your active theme and click on “Customer Login” in the left menu under “Checkout Pages”
Users do not have access to edit the default store text. You will need to create a new store text entry. To do this, type a name in the Localization Name textbox and click the yellow plus sign. From there you will be brought to the edit page for the new Store Text you just created. Once you have edited the section(s) that you need to simply click the Save button.
Once saved you will need to open your store setting, and set the "Active Store Text & Language" by clicking on the drop down next to it and selecting the name of the store text that you had just created and then click save.