One way to do this is through code (of course). In the Form you have the textbox (we'll call it txtbox1). The code should probably be something like this:
dim strTableName as string
dim strSQL as string
'This will get the string name of the table
strTableName = me.txtbox1.value
'This is the SQL statement that will change via the user's input
strSQL = "select * from " & strTableName
Now you can do whatever you want with the query. Something like the following
[form]![FormNameThatyouwantchanged].recordsource = strSQL
This will change the record source of the form to the query that the use specified.