i had using 5kg dumbbells from the beginning of my workout for about 1 month , den i decide to change the weight to 10k .... and it was very hard to lift .
Do i need to change back 5kg or continue to use 10kg?

bradder... u suddenly jump 5kg of course right....

7 ftw!

thx for the lecture bro.
if continue workout with 10kg will injured my self?

u will be recruiting other muscle group to lift that weight if u cont....
which may defeat ur purpose of building a specific muscle group

You may not trigger the muscle you targeted to, or even will not be a proper form. get 7.5 or get a spotter to help you. Make sure you're in proper form.

get 7.5kg or 5kg but increase the reps

increase the weight slowly.....from 5kg to 7kg and then to 10kg.....