How to send an email using Rules when a node's boolean field is changed?
Here are some details about how to make this work (using Rules):


Rules Event: Before saving content.
Rules Condition (1st): "Entity has field", whereas you should specify these values:


"node" (saved content) for the "data selector" of entity.
the machine name of your boolean field (for field).

Rules Condition (2nd): Compare the value of the specific field, for both the original value (before editing the field) and the (new) updated value, to verify if they are different.


So part of the "clue" to make this rule work is the condition Entity has field. That's also what is mentioned in the Rules UI when you're adding a "data comparison" condition: somewhere it says "The data selector helps you drill down into the data available to Rules. To make entity fields appear in the data selector, you may have to use the condition 'entity has field' (or 'content is of type')". For a video tutorial that illustrates the importance of this Entity has field condition, refer to Data types and data selection, especially what is shown between about 13:30 and 17:30 in that video.

Update (to further clarify the new questions in the "update" part of your question, with the quoted text copied from the update in the question):


  I am not sure what to pick at Data selector, while at bottom (Field: Value) i found the field I need.


You should select node (saved content), because you want to check if a "node" has the field you're looking for.


  ... but also did not find an option: "Rules Condition (2nd): Compare the value of the specific field, for both the original value (before editing the field) and the (new) updated value, to verify if they are different."


For this you add an additional Rules Condition which is a Data comparison. And the actual data to be compared are:


original value of the field, which you should be able to find/select by first using as data selector the node-unchanged:...(unchanged content), followed by selecting the field name of your field from the additional list that is shown after you selected that node-unchanged:...(unchanged content).
updated value of the field, which you should be able to find/select by first using as data selector the node:...(saved content), followed by selecting the field name of your field from the additional list that is shown after you selected that node:...(saved content).