Select the INCORRECTLY spelt word.

The correct answer is swetch.

Let us analyze each option to identify the incorrectly spelt word:
1. glimpse: This spelling is correct. A glimpse is a brief or quick 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 a change from one thing to another.
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 among the given options is "swetch".