To change a linear sequence to a circular sequence, and vice-versa, select the sequence and choose Sequence->Circular Sequence(Linear Sequence).
If you already have a circular sequence but want to view it as linear, use the advanced option Linear view on circular sequences, shown in the screenshot below. This option does not convert the sequence to a linear sequence document, it only changes the sequence view.
While it is possible to view a circular sequence in the linear view, you will not be able to view a linear sequence as circular unless you convert the sequence using the Circular Sequence option.