how can i convert this?

A pocket calculator (computer, mobile phone, etc.) should do it. A minute is a sixtieth of a degree, so take the decimal part of the position in degrees and multiply it by 60 to convert it to minutes. Thus in the example that you gave, latitude 42.358372° becomes 42° (0.358372 x 60)', or 42°21.502'.

Next, you need to know that negative latitude means South of the equator, and negative longitude means West of the Greenwich meridian. That is simply a convention adopted by all geographers.

So your position becomes N42°21.502' W8°51.485'. That is the format most commonly used for navigation at sea, where it is useful because a minute of latitude is a nautical mile. The same format is used for geocaching.

You can, if you wish, convert the minutes to minutes and seconds in the same way, as a second is simply a sixtieth of a minute. Expressed in that way, the position that you used as an example is N42°21'30" W8°51'29".

It's in the coastal waters off North West Spain.

How hard could it be?

sLat = (lat -lat%1) + "° " + (lat%1)*60 + "' " + (lat>0?"N":"S")
sLon = (lon -lon%1) + "° " + (lon%1)*60 + "' " + (lon>0?"E":"W")

It is not like you need proj4js or anything when you are just converting the display style.