There are several ways/places to apply passwords. Here are the most common
2:

You can apply a database password that users must supply when opening the
file.
(File must be open exclusively to make this change. Open Access

Dialog and select "Open Exclusive")
Then, in Access UI: Tools>Security>Unset Database Password: supply current
password

You can apply a password that would prevent changes to Code (and to
forms/reports that have code attached).
In the VBE: Tools>Properties>Protection: uncheck "Lock Project for viewing".
optionally, delete password.

A password can also be required by incorporating User-Level security, but
that's a major undertaking so I'm guessing that isn't what you did (and I'm
not familiar enough with it to tell you how to undo it...)

You first need to open the database exclusively.

Open the database exclusively by opening just Access.
Go to File, Open and search for the database in question.
Click once on it in the Open dialog box.
Then click on the dropdown next to the Open button.
Select Open Exclusive.

Another option is to open the database exclusively with a command line
option. Put something like below in the Start, Run dialog box with the proper
paths and file names. The /excl opens it exclusively. If you do this often,
you might want to make it a shortcut.

"C:\Program Files\Microsoft Office\Office11\MSAccess.exe" "C:\Program
Files\Microsoft Office\Office11\Samples\Northwind.mdb" /excl

Next go to Tools, Security and you should be able to remove the database
password.