by corvus albus.
Something went wrong with formatting. Hope it works now. Replace <entryid> und <filedid> by the corresponding ids.
https:///webservice/rest/server.php?
wstoken=&
wsfunction=mod_data_update_entry&
moodlewsrestformat=json&
entryid=<entryid>&
data[0][fieldid]=<fieldid>&
data[0][subfield]=&
data[0][value]="<p>html-text</p>"&
data[1][fieldid]=<fieldid>&
data[1][subfield]=content1&
data[1][value]=1