Click on the icon that represents the account signed in (the circular graphic shown in my screenshot above) to open the menu in the screenshot.

Click on the name of the individual signed in - this will open up a new window over the top of the Store.

Click on the account again to be provided with a "sign out" option.

Then click on the icon clicked on in the first step (now represented with circle with a line sketch of a head with shoulders).

Click on "Sign In".

This will provide options for signing in as the original account or as a different Microsoft account.

Note - any account that has been signed into the store within this logon to the machine will be available for use in the store on this account from now on without needing to provide a password. Example: My son logs into the computer, I log him out of the store and log in to my own account. I can then go to my Library, tell the store to install each of the apps for this logon account, sign out as myself and sign him back in. When I sign him back in, a password doesn't need to be provided. If I need to switch back to my account to link something else (say I forgot to link something), I can sign him out again, and when I click on my own account to sign in, I do not have to provide a password.