At the end of a reporting period, how do I record increased/decreased share values?

For personal accounting (i.e. your own "balance sheet") you could create an entry for "unrealized gain" that you would then need to clear out when you sell the stock and it becomes "realized". You'd also have to decide when to account for unrealized gains - once a year? Once a month? Or just leave the value at cost and track unrealized investment gains elsewhere (since it doesn't really matter until you realize the gains)

Businesses typically do not record unrealized gains in the balance sheet except in certain circumstances, the details of which are off-topic for this forum. (search for "Cost Method and "equity Method" for details)