The ArcMap Editor's Edit Sketch Properties dialog provides access to editing the individual M values that make up the geometry of a polyline feature.
1. Start editing a polyline feature class.
2. Select one feature.
3. Change the Edit Task to Modify Feature.
4. Right-click the feature and select Properties to open the Edit Sketch Properties dialog.
Modify the X, Y and M values for each vertex in this dialog.
Edit the M values for polyline features by using the advanced VBA capabilities of the ArcMap Field Calculator. See the link below.