Hi Arcadinal,
In Fit Y by X the confidence level for the parameter estimate confidence intervals is not selectable. However, you can perform the identical analysis with the more flexible Fit Model and in that platform you can set the confidence level for the confidence intervals. To set the alpha level, first launch Fit Model (Analyze > Fit Model) and specify your Y and X variables. Before clicking Run, click the Red Triangle in the model launch and select Set Alpha Level. To get 99% intervals, enter .01, for 90% intervals enter .10.
Next, reveal the confidence intervals in the same way you did before (right clicking the Parameter Estimates > Columns). You will find confidence intervals corresponding to the alpha level you specified.
I hope this helps!
Julian