You have several options to do this, but it kind of depends on how the annotations were set up and how much you want to control every aspect of those P&ID's going forward.
The quick and dirty way would be to change the font associated with the different textstyles.
Type the command STYLE and for each text style, change the associated font to the one you wish to use.

All texts and annotations will use the new font. You would need to repeat this for each drawing individually. Also do this in the project template used for P&ID's so that your future P&ID's use the correct font.
If you want more control over the naming of styles, you can make your own style using the font of your choice. But then you'll need to update each text/annotation object individually to use your new style.

In some cases that will mean that you'll have to edit block definitions (use the BEDIT command). For example the hand valve annotation (like HA-104 in the above screenshot) is a block definition:
- select the texts (Attribute Definitions) in the block editor and change their style there:

If you really want it very clean for the future, you can also change the project setup. Go through the different annotation blocks in your project setup , and change each one in the project setup (or change it directly in the projSymbolStyle.dwg of your project)

This way all future labels in your project also use the correct style/font. Existing labels will have to be updated manually as described above. You will need to repeat this for each type of annotation used in your project.