records numbered sequentially

This is most likely a number field with an auto-entered serial number.

Open Manage | Database | Fields

Find the field's definition and double click it.

Click the auto-enter tab.

Now you have a dialog where you can change what will be entered in the next new record to be created.

However,

I strongly advise carefully examining the structure of your database and how this field is used in relationships. This field might be used as a primary key for uniquely identifying records. Altering this value, as you've already discovered in one part of the database, may affect how data in table links to related data in other tables.

Be cautious and make a back up before you change anything.

What version of filemaker are you using?

If this is a hosted file, some versions won't allow access to this section until you take it down off the server--that's a very good idea here anyway since you don't want the changes you are making to impact actual use of the database until you are sure your changes will not cause problems for the database.

Also: You need to have the Database open with a full access password and there shouldn't be any paused scripts. (You'll see continue and cancel buttons in the status area if a script is paused.)

If you were not asked for a password when you opened the file, it is still possible that a password was used to open the file for you when you opened the file. Open the file while holding down the shift key, Windows, or the option key, mac, and see if you are asked for a password.

It appears your password is not a full access password. Restrictions on your password's privilege set are likely preventing you from accessing Manage | Database. You should contact the last person to make development changes to the database to get this password.