My ex husband altered our divorce decree without my consent. The amount of spousal support each month and length of time. Is this legal?

It all depends upon how he "altered" it.  If he just unilaterally wrote in something else on a copy of the decree, of course it's not legal.  If he petitioned the court for a modification of his spousal support, served you with a copy of his petition in a legally correct manner, and if you did not respond by filing an answer with the court in the time required by law, you may have "defaulted," allowing him to obtain the modification without your consent.