Select the INCORRECTLY spelt word.

The correct option is "swetch".

Let's analyze each of the given options to find the word that is spelled incorrectly:
1. glimpse: This spelling is correct. A glimpse is a quick or temporary look.
2. swetch: This spelling is incorrect. The correct spelling is switch, which refers to a device for making and breaking the connection in an electric circuit, or an act of changing or swapping.
3. achieve: This spelling is correct. To achieve means to successfully bring about or reach a desired objective or result by effort, skill, or courage.
4. calendar: This spelling is correct. A calendar is a chart or series of pages showing the days, weeks, and months of a particular year.

Therefore, the incorrectly spelt word is "swetch".