If these are regular (non-Split) transactions in a banking account register, you can try to use Edit Menu / Find/Replace to find the desired transactions. Review the Find results, select the ones to change and then Replace Tag with A-5
Be careful when making changes to a number of transactions at once. Undoing a bad change may be difficult. Therefore it's best to have or create a backup of your data file before you start making changes. If things go bad, you can restore your backup file and start over.