Its very simple...... If it hasnt been maintained well (look at your maintenance records for that one), then Id do a flush and a Tstat change at the same time......
My method is to take out the Tstat, drain/refill the radiator with distilled water, keep going until almost clear water comes out, and then do an appropriate fill........ Then right before the final fill, put the Tstat back in.......
If youd like more detail, do a search..... I wrote a TON about this sometime earlier........
If you dont want to do it that way, you can take it somwhere to have it done...... maybe $50 or so.......