To change the position of the origin of your circular sequence, or to change the numbering on a linear sequence, select your sequence and go to Sequence->Change Residue Numbering.
On a circular sequence this function can be used to shift the origin of the sequence to a different location.
On a linear sequence this can be used to indicate that the sequence is a subsequence of a larger sequence; to number a sequence with respect to a particular location (ie make the start of a gene base 1); or to reverse the numbering of a sequence. This will introduce two numbering systems into your linear sequence: the original numbering (Standard), and the numbering that you have specified (Source).
For example, the sequence below has been renumbered according to its position on a larger scaffold, where it is in the reverse orientation between bases 9740 and 25477.
The new numbering is shown underneath the original numbering.