how can I copy a sentence in lowercase, and paste it in uppercase?

Hello its_just_tzviki,

From your post, it appears you are looking for a way to paste a sentence and have it changed to all uppercase.

Typically, each program would have its own way of making a conversion like this.

For example, in Safari, once text is pasted, if you highlight the text, then click Edit > Transformations in the upper left, there should be an option to "Make Upper Case."

Have a great day.